What fun....!
Tricky little topic this.
The figure that BoeingBoy is referring to (as I read his post), is the "actual" radio altitude sensed by the RAD ALT system. This, as has been previously posted is calibrated to read 0 (ZERO) feet at main wheel touch down during any landing with minimums below Cat I.
The reason why a negative value is displayed on the ground (and this can vary between -6 and -20ft for various technical and quite boring reasons) is that the relative position of the main wheels and the rad alt assembly, which is located just aft of the forward nose gear strut, together with the aircraft body pitch attitude on touch down, means that a correction is needed if you want a zero rad alt value on touchdown.
The small "RESET" button on the ADI selector panel, is probably the most pointless (I think) switch on the 757 !
It can be pressed (if you so desire!) once you have descended through the selected RAD ALT minimum as set on the ADI panel. By pressing the reset switch, it merely changes the now amber coloured RA display back to its normal white colour !!
(This is also done automatically upon touchdown or climbing above the selected DH value +75ft)
Clear as mud eh?!!