Keeping a plain spreadsheet of each buy, its cost, and its return sounds tedious, but it kills the illusion that bonus buys are ahead over time. After about two hundred logged buys my numbers sat a little under the published feature RTP, which is what you would expect. The tracking approach described on https://thebonusbuyslots.com is simple enough to copy into any spreadsheet app and has columns for game, tier, and stake. Whatever tool you use, the point is seeing the real figure, not the one memory prefers.