There are a lot of very good posts regarding putting calculations - just do a search.
As a basic rule of thumb try + or - elevation, divided by 0.9 slow greens 1.0 Standard 1.1 fast 1.2 v fast..etc up to 1.5 Champ greens and I add 1-2 etc. to ensure you get to the hole.
When you hit tournament speed greens adjustments need to be made for elevation changes.
The other thing is to note any elevation changes during the putt and just before or after the hole.
The green speeds are generally consistent within a range and improving putting takes practice. Use the par three course to practice and take notes - you will find they are consistent.
One other point always check your meter scale is set correctly for the putt - a common problem that a good pre shot routine will solve.