So basically, there are two ways I could see this, but first, I'll explain the shared way this works.
Basically, take what the Ability Modifier is for an Ability Score that is an odd number, and add 0.5 to the modifier. For example, a Strength Score of 13 is normally a Strength Modifier of 1, so under this change, it would be 1.5
Now onto the two ways this could be done.
1: Partial. You only round down AFTER calculating everything, and you recalculate everything whenever you gain a level like normal. For example, a Con Score of 15 would give 2.5 bonus hp per HD. Your first HD would give 2.5 bonus hp, rounded down to 2, then your second HD would give 3 hp, because 2.5+2.5=5, and you already got 2 points of that 5 from the first HD. Another example, with the Str Mod of 1.5 from the Str Score of 13, if you wield a one-handed weapon, you get +1.5 damage, which rounds down to 1, but if you wield that weapon with two hands, you get 1.5 times Str Mod, and 1.5*1.5 is 2.25, which would round down to 2 damage.
Another example, if the creature takes extra damage from that damage source (say, via vulnerability) you would calculate the extra damage to get the full damage BEFORE you round down.
2: Full. Simply remove the concept of rounding down altogether. Makes math a LITTLE bit more annoying, since now you are always dealing with stuff after the decimal point, but I mean, if you are playing Pathfinder 1e for fun, you probably don't mind the math.
Anyways, does anyone have experience with this? I don't know if I actually want to play like this, just sometimes I like to imagine a hybrid of real life with Pathfinder, like maybe applying a template to real world humans or something, and well, there is no reason in real life that math would just magically round down like that just cause something went past the decimal point, so it got me thinking about possibly actually playing like that in Pathfinder.
