Milling (or skimming) the top of the sleeve to help raise compression isn't really a good idea because the thickness of the flange can be considered to be a strengthening rib to help keep the top of the sleeve round. Engines will run just fine even with zero nitro and stock compression but stock compression is way too low to take advantage of the compression that methanol can handle. This just means that the fuel isn't being burnt as efficiently as it could be and that's one of the reasons for less power on low or no nitro. The best (just about only) way to raise compression to high levels is by machining away the sealing area of the head/head button to get the head closer to the piston at TDC. But not too close . Some engines though need more machining than just the sealing area, it all depends on the initial set up.