It's been done, some computer overclockers or reviewers have built quite elaborate test jigs to compare various thermal compounds.
But testing for optimal amount is much simpler: remove the heatsink, check if the CPU is fully covered. Anything more than that and it will be squeezed out under pressure. The main difference is how much cleanup you will have to do the next time ![Tongue :P](https://www.eevblog.com/forum/Smileys/default/tongue.gif)
The eternal question is: pea or grain of rice ![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
For that size, grain of rice, place and press firm, and see how much coverage, and then apply a little more so it makes for a fill coverage.
If you need more than that your heatsink is warped, so take it onto a surface plate with some 1000 grit waterpaper, wetted with water and a drop of detergent, and lap it to an even smooth matt finish. Then clean, degrease and try again, it makes a big difference. Also make sure the CPU itself is clean, grease free and that no dust or other debris falls on the surface before you place the 2 surfaces together.
I had a lot of older Pentium CPU's where the original fan died, and there was no replacement that would fit the mounting, so replaced the heatsink complete with a much bigger one, held to the CPU with metal filled epoxy. They ran a lot cooler than the original, simply because of the larger area of fins, and improved airflow. Just was going to be difficult to ever remove them again, as the new unit would foul the lever lock for the socket, but those boards were in almost every case going to run till they died from capacitor plague anyway.