Take this data set of mine:
Lv: 9
Trait: N/A
M.Dmg: 14
MND: 34
DTR: 34
minHP: 64
maxHP: 67

For "HP=round(3%±RHP)" you have:
RHP*0.97≥63.5 because round(63.5)=64, and
RHP*1.03<67.5 because round(67.5)>67;

so you have:
RHP≥65.4639, and
RHP<65.5340.

You can therefore write:
65.4639≤RHP<65.5340, or more accurately
63.5/0.97≤RHP<67.5/1.03.

That gives you a range of all possible RHP values for that specific data set given that HP=round(3%±RHP). Then you adjust a value of your choice (I do that with MND) and just keep gathering more data and RHP ranges, then you take all the minimum RHP values and graph them, and do the same with the maximum RHP values (or rather threshold RHP since RHP must be below that value). Check if any progression that makes sense (linear, exponential, whatever) is possible given that range. If no progression is possible, the construction is proven false. If all the aforementioned constructions are proven false (I'm starting to lean to that with the data I currently have, but I don't want to jump to conclusions just yet), the formula is likely something such as:

HP=round(3%±round(RHP)/2),
HP=round(3%±round(RHP)/3), ...
HP=round(3%±floor(RHP)/2), etc.