I've mentioned this idea a few times on here and discussed it in PM with several authors here. Nobody knows what Amazon's algorithm looks like. There's a lot of speculation as to how much and how long historical sales contribute to ranking. That aside, we know one thing with absolute clarity. It's mathematical. It's numbers. Regardless of how much weight a two day old book sale carries or how long it will still matter, a perfect promotion can still be easily described.
Since ranking is updated every hour, nearly everyone agrees the algorithm is hourly based. The primary contributor obviously would be the most recent hour's sales and each previous hour is weighted as some percentage of that multiplier. Whatever the length, in hours, days, weeks, or anytime period, each previous block of time in this constantly moving block of time is recalculated hourly, compared to every other book on Amazon and given a rank based on that comparison.
Being a numbers guy, it was obvious right from the start how a perfect promo would look. Regardless of how many books I'm currently selling, the promo will achieve the highest possible ranking, if every single hour of the five day period had a higher sale. Actually, perfect would be a higher number of sales by the same amount. In other words, when the promo starts and my book sold 2 copies in the last hour, I would want the next hour to have 3 sales, then 4, then 5, and so on, culminating with 122 sales in the last hour after 5 days ((5x24))+2. That would create a perfect 45* incline when charted.
It's simply not possible to set up a promo that will do that. There's no way of predicting the main variable, which is the reader. Some days book sales just stink on ice and subscribers don't open emails. Therefore the perfect promotion isn't achievable. But, the closest you can get to that perfect 45* incline, the better your peak ranking will be.
Getting your book higher in rank is the ultimate promotion. At a certain point in the promotion, your book is likely to be #1 in several sub-genres. The higher it climbs the less control you have with advertising, which creates a whole new uncontrollable variable.
The best promo you can set up for a discounted book, whether it's in Select or not, would be to maximize the affects of the ads you buy. You want to set your ads up in as close to a perfect incline as you can intelligently guess they'll create. Putting a lot of ads on the first day, that all post or email early in the day, then having only one ad in the afternoon of the second day, will cause a steep rise, and precipitous fall.
I subscribe to dozens of advertisers, getting recommendations emailed to me every day for books in my genre. Have you noticed that you get them about the same time each day? Oh, sometimes one will be twenty minutes later, or an hour earlier each day, but they're pretty much in the same two or three hour block each day.
First, choose your anchor ad. If that's BookBub, they send their emails between 11am and 12 Eastern time. To maximize the ranking of the sales from BookBub, you should have another big ad, or several smaller ones earlier in the day, between 6 and 10. To maximize the affect of that one, a slightly smaller one in the four hour block before that. It gets difficult in the overnight hours to find an advertiser that emails at 2 am. But then, sales are slower overnight. But, think outside the box. If it's midnight on the east coast of the US, it's early morning in the UK and afternoon in AU.
By setting up your ads so that each block of time has enough ads to surpass the previous block of time, you can straighten that line out a little and get it closer the the unachievable perfect line.
Recently, Readers in the Know announced a new marketing tool (www.readersintheknow.com/list-of-book-promo-sites
), with literally dozens of advertisers that you can sort by all types of criteria. Simon reached out to me and asked my opinion. I'd already seen it a few days earlier on Jan Hurst-Nicholson's thread and liked how it worked. I explained my idea of a perfect promo to Simon and he liked it, adding another column to the tool that you can sort by email time and change to your own time zone. I gave him the list I'd compiled of advertisers I use and he added those times and is now contacting each advertiser on the tool, to get their email times.
That can help a lot in picking the right advertisers for a particular time of day. But, you should still do your own research by subscribing to them. Some may email different genres at different times of day.
*Edited to fix the link.