bit of advice needed

[oldskoolron]

so i have a seig sx2 mini mill with the 500w brushless motor .it does wat i need it too nearly all the time but speed control is hard as it doesnt have a tacho just the dial you turn from zero to about 2500rpm .advice from you fellas is how can i work out speed for the cutting tools im useing ive tried turnin the dial half way for face cutting 6061 but to me it seems to slow but im have only 30% hearing in both ears so cant really judge by ear .am i on the right track just divideing the amout of dial turn into say 5 parts an useing each ingrement as 500 rpm an so on
cheers in advance