HP and ATK correlation script/formula?

 I'm trying to find a way to have the percentage of HP that the user has correlate with how strong their attack is.

Ex.- Player A has 100% HP, so they use 100% of their ATK skill in the standard formula (a.atk * 4 - b.def * 2). However, if Player A has only 47% HP, they only use 47% of their ATK skill in the standard formula. 

I've been toying with it for two days, looking up different posts and articles, and I can't find a way to correlate the two. If this is too complicated, I would be completely okay with this: a.atk is 100% as long as a.hp is 51%+; a.atk is 50% as long as a.hp is 49%-. (I also have an idea for a skill that would inverse this, i.e.- skill that does more damage when HP is critical).

I hope this wasn't too much information. Can anyone help me out?

((a.hp / a.mhp) * a.atk) * 4 - b.def * 2 should do it. The a.hp / a.mhp calculates the percent of HP you have, then takes that times your attack, then multiplies that number by 4.

Edit: There is a chance that might give you 0 when the HP is < 100%. If that happens let me know, we will need to convert the value from a.hp / a.mhp to a float to use it then.

Thanks a ton for your help. I was testing it, and as soon as the HP< 100%, attacking didn't deal any damage, like you said.

Try this:

((a.hp.to_f / a.mhp.to_f) * a.atk.to_f) * 4- b.def* 2

This worked perfectly. Thanks a ton to both of you for your help!

I thought that the .to_f might be needed, but wasn't sure. Thanks whitesphere for fixing that!

to_f is only needed for the division. And only one of them

Or you could simply have used the formula

The to-float is only needed if you need to carry values below 1.00 into the next part of the calculation - if you do the division last, it is not needed.