There isn't a handy formula for calculating a reasonable CYA. A higher CYA even with a proportionally higher FC, loses less absolute FC per day (this is shown in the table in
this post but of course the absolute amounts will vary depending on specific conditions of temperature, amount of sunlight, bather load, etc.). If that were the only effect, then a high CYA would be the way to go. However, at a higher CYA level if anything goes wrong and you need to
SLAM the pool, it takes a lot more chlorine to get to shock level for the higher CYA level.
So the general rule-of-thumb that we use is that somewhere in the 30-50 ppm CYA range is reasonable for most non-SWG pools. Pools with more sunlight can be at the higher end of the range and pools with lots of intense sun may go even higher, but with the risks well understood. Pools with no sunlight, including indoor pools, can go lower to 20 ppm if they want but 20 ppm is the lowest measurable on some CYA tests (yet other CYA tests only measure down to 30 ppm). For SWG pools, 60-80 ppm with 70-80 ppm preferred is the usual recommendation because one wants to minimize SWG on-time to minimize the rate of pH rise and to lengthen SWG cell life. Since the dosing is more continuous and automatic, it is less likely to run into problems with an SWG pool and therefore less likely to need to shock to high FC levels. So the risk of the higher CYA level is a lower risk (i.e. less likely).
With full sunlight on your pool, why don't you start with 40 ppm CYA and see what kind of daily chlorine loss you have. If it's acceptable, you're good to go, if it's too high, then raise your CYA level but understand the risk you are taking the higher you go. There's no problem so long as you properly maintain your pool, and that of course requires a proper test kit. If you go with the supplemental algae prevention at extra cost that you seem to be interested in, then you lower your risk of a problem so a higher CYA would save you money on chlorine due to lower daily chlorine loss. It's a tradeoff between the extra cost for supplemental algae prevention vs. the cost of chlorine. It's not what we normally recommend because it's more complicated and doesn't apply equally to most pools (i.e. which approach to take depends on other factors like phosphate level, cost of products, size of pool, amount of water dilution, whether some products are already being used for other purposes such as borates, etc.), and can be more expensive for some pools (those with very high phosphate levels, for example).