Thermal grease (used on CPU) is not a substitute for a thermal pad (used on NB) and required size/thickness should be documented for each thinkpad as it's not documented anywhere else (including HMM). Bad thermal pad will cause NB to throttle down, which will show up eg. as lags on video output (since GPU is on NB).
If you are going to measure it, use unsquished section that wasn't damaged/stretched out with calipers
x200s: 0.5mm Gelid Solutions GP Extreme pad used for NB with good results.
Required size is very small (perhaps 15x15mm approx).
"unsquished" thickness of original thermal pad measured approx. 0.5mm.