CompuServe Messages

I have a 486DX2-66 with a Diamond Stealth 64 VLB card with 2 MB DRAM. I use it with a 15' CTX monitor, and for some reason cannot get it centered correctly in DOS after having centered it in Windows. I have been using User-defined monitor settings, starting with Windows at 800x600x256 and then centering Windows with the monitor adjustments. After doing this, I quit to DOS, and run s64dmode.exe, select Custom Monitor at the same resolution with polarity +/- as per the monitor manual, and then center the rectangle in the centering utility. When I exit the centering utility, the screen is immediately off-center, and stays that way for all DOS programs. Windows remains centered. Am I doing something wrong? Thanks in advance.