To set the mood for this answer, we should all remember how this works. Clear your mind, and will the paper to burst into flames.
Now, let's start with two assumptions:
Your world doesn't have access to thermometers, much less any more precise technologies.
You're looking to create a "hierarchy within a guild," meaning a means of comparing one practitioner to another ... not necessarily a way to say "that guy can produce 2,000 joules/second!"1
Under these assumptions your easiest solution is for your well-developed, respected, and ancient guild to have selected a half-dozen materials with specific quantities. Let's say (somewhat arbitrarily)...
- a 1/2 cubic ft block of aspen wood (how long to burn to ash),
- a gallon of water (how long to bring to boil),
- an ounce ingot of copper (how long to melt),
- a 1# ingot of steel (how long to melt),
- A 5# block of glass (how long to melt),
- a 10# block of obsidian (how long to melt).
The effort, because there are sub-achievements (like cracking the glass, warming the obsidian, etc.) would provide a basic gradation of skill and power. An initiate might be able to get the wood to burn, the water to steam, and the copper warm. An archmaster might be able to vaporize everything in under an hour.
This method is imprecise, especially since weather is a factor. The job is simpler (if only fractionally) when done on a hot summer day than the dead of winter. But, it would create a viable pecking order.2
So, let's change our assumptions:
Your world has access to thermometers and stopwatches.
Practitioners carry around government-mandated identity cards that list the statistical capabilities of the practitioner somewhat like a passport.
Under these assumptions your solution isn't much different, but demands fewer materials. Probably just one. Let's say the goal is to see how much the practitioner can raise the temperature of a sphere of Inconel with a radius of 0.5m having a mass of approximately 4.3kg in 20 seconds. What we're looking for is the measure of ℃/s.
This measure (℃/s) has, of course, become known as the Naidoo after the creator of the measure for pyrokinetic purposes, practitioner Iminathi Naidoo. The Naidoo reflects the potential of the practitioner to heat or combust anything. The ability to raise the Inconel sphere 20℃ in 20 seconds means your Naidoo rating is 1. But, what does this mean?
I'm glad you asked! The "Naidoo Combustibility Index" or NCI (the measure of how many Naidoo are required to "spontaneously combust" or bring a material to flame in one second or less) was developed and referenced by practitioner Naidoo in the NCI Reference Guide.
Look at it this way. The thermal conductivity of Inconel is (basically, there are a lot of kinds of Inconel) 73 kW/m℃. That of copy paper is 0.33 kW/m℃. This means you need 221 times the Naidoo rating to heat paper as fast as you heated Inconel.3
But, the NCI is a measure of spontaneous combustion. Paper's autoignition temperature is between 218℃ and 246℃ (call it 230℃ for fun). With a room temperature of 25℃, we need to lift the temperature 205℃ in one second or less. Thus, the NCI rating of copy paper is 221 * 205 = 45,305 (let's call it 45,000 Naidoo or 45kN4).
Now, if you can't spontaneously combust a piece of paper, then you have no business joining the Pyrokinesis User's Guild (PUG), minimum of 45kN to join.5
1 Not to be confused with Watts, right?
2 "You mean you couldn't melt the copper? Hahahahahaha!"
3 Oh, I'm simplifying this process something awful, but I'm having fun doing it.
4 Anyone thinking that N means "newtons" is being way too serious about this....
5 And I am having way too much fun putting the "fiction" back into "science fiction."