Monster level up patch

I'm working on a monster level up patch. I started working on this when I was doing work on my hack but never finished the idea. I took the idea from chillyfeez on Slick Forums who has implemented the idea for his FFIV hack. I need some input on balance with the system I'm planning.

First I calculate the average party level (APL) and multiply it by 2/3, which gives the modified APL. For a average of level 18, APL is 12.

Then I calculate an adjustment ratio by doing modified APL * 100h / monster level. For a lvl 8 monster, ratio would be 0Bh * 100h / 08h = 160h

Then, if we take a monster stat, we do stat = stat * ratio / 100h. For a monster HP of 48, we would do monster HP = 30h * 160h / 100h = 66 HP. That would be his HP for the fight.

Obviously not all stats can be adjusted that way (thinking about speed for instance) so I'd like to know what should be tweaked with the base formula and what could be tweaked another way, preferably with the modified APL or ratio. I'm thinking excluding experience and gold from this. All stats that are 0 will never raise. The thing with the calculation for level is that the monster new level always en up being the modified APL. I haven't calculated for a monster who is a higher level than you but his stats and level should lower. If someone foreseen a problem with playing with monster levels, please let me know.

I'd like to release a nice system that gives more challenge than vanilla. Obviously all monster stats need to be tweaked, but after that you could only adjust the monster level to make him harder or nerf him. I plan on releasing a separate patch that balance the monster stats to the new system.