You a fortune teller?
The reserves game is just that, a game, I've seen engine waaaay past TBO still make great numbers and I've seen low time engines crap out and everything in between.
I've always just calculated the normal predictable monthly stuff, like hangar, insurance (if needed), Jepp if I'm going to be doing much IFR work, and my normal flat rate for the owner assist annual.
Aside from that I check oil, put gas in and go, if something comes up along the way I nip it in the bud, but I'm not putting money aside for engine and avionics and whatnot, heck a AI overhaul ain't much money and a OH on a 520 is the most expensive thing I'd be looking at and when that time comes I'll just pay it and maybe tighten my belt for that year, whatever.
When you do these types of "reserve" calculations it makes aircraft ownership, heck even car ownership near impossible, with the numbers you use I'd really have to budget to even go rip around the pattern, yet somehow even though I don't put millions away in a special account for each flight hour, my aircraft stays in top shape and my mechanic is more often telling me I'm going overboard on stuff vs being able to come up with any significant snags, outside of a bearing or a tire here and there.