Try this rule of thumb, keeping in mind that when the uphill or downhill inches are displayed, they are rounded off. Downhill putts should not be affected because a little extra is better than too little.
But an uphill putt of 1 inch can be 1.4999999 inches and still read as "1", so on uphill putts, do not be afraid to add and extra foot to your putts.
Fast greens would putt at 100% downhill and the same uphill, plus that extra foot!
Actual distance plus uphill inches equals the real distance you must putt, then add that extra one.
VF greens (all else being under the same basic rules), will put .875 downhill and .90 uphill plus that one extra.
Tournament greens will putt 75% downhill and 75% uphill plus one.
If the slope is more severe than 3 ... there might need to be secondary adjustments like another foot or two uphill and one less foot downhill.
At four inches and greater (in slope), it gets dramatically different. Only experience can guide you here, especially on Olympic or Merion!
There is no spin meter for putting, but severe tailwinds can make ball go a fraction longer whereas severe face winds will slow them down, but not on tap-ins.
ALWAYS hit tap-ins with at least an extra foot unless severely downhill.