How to set exact rates with Powerstrip

[Serpent]

I have a Geforce 1 DDR which I use under win98SE. I've noticed that when I select 60hz or default refresh rate with display properties, it's not exactly 60,000, as I can verify with Powerstrip, but a number more or less close such as 60,100 etc. I think this is the cause of my MAME not being totally smooth all the time. Using Powerstrip I'e been able to come closer to 60,000 but never to the exact number. I don't know how to do this because I don't understand the parameters I'm adjusting, and the only thing I can do is test and test, so it's a matter of luck. With my other computer, where I have a Radeon 7500, Powerstrip reports 60,000 as default. I've noticed that the pixel clock and scan rate values there are exact numbers: 162,000 and 75,000. I cannot get these exact values with my Geforce. Could it be a problem with its drivers or the monitor drivers, or anything of windows or powerstrip that I've not set up correctly?