Maybe just divide the number of humanity points you get by ten, and keep track of fractional points? I think that would be easier than incrementing a tick every turn.
It does assume you're going full strength or full speed, because in general that's smartest. Dodging gets more useful the better you are at it (+10% chance to dodge is more meaningful stacking with 80% chance than 0% chance), as does attacking with either stat. To ensure even one multiattack, you need 99 spd minus the speed bonus on your attacking limb; since part stacking is so heavily penalized, it's harder to justify trading a strength part for a speed part. Multi-attacking comes as a costless bonus to a spd build, but is very expensive for a str build. We aren't being given too many points, because there's also int and heart, and you can only really make two stats have a high level. Make yourself into a combat juggernaut? Great, good luck attaching limbs with your low heart and int.
Hmm. I am assuming it's best to ensure you can land all your multiattacks. For a speed build, that's true, as you're only increasing damage by 15% if you get a seventh, but a str build getting a second multiattack is doubling their damage. Against another strength user, it's a bad idea if there's any significant chance of failure, because they deal their normal damage plus fifty if you fail, but against a speed user they're just getting one more scratch in. Hmm. That might tip the favor a bit more towards strength, as it's essentially passive defense against people taking a risk against you. I still think spd is better though, for the utility of multiple attacks and dodging. I'm not gonna actually do any math before you finish the system, at least, because there could be compounding mechanics which completely change things.