If you have pH measurement strips, or a pH kit with chemicals, you can do a measurement of pH of compost, similar to a soil pH test. The procedure is called a slurry test.
Take equal weights* of (a) distilled water and (b) the compost (or soil). In a plastic or glass container, thoroughly mix the water and compost together, then wait 15 minutes and mix again.
You can try directly reading pH of the wet slurry. You can also put the slurry into filter paper (such as a coffee filter) and test the liquid that comes through. That is your starting pH.
Measure the mass or volume of some compost you want to pH-adjust, and the mass or volume of lime you want to add. Do the same pH process over again, except I would wait a good while after mixing in the water before measuring pH. Maybe 2 or 3 hours?
The rest requires a good bit of "fiddling around". After pH testing, do you need to mix in more lime, or do over with less lime?
[*I would do with a small mass of compost and water, like 25 grams of each; if you don't have a scale, do by equal volumes of each]
The RHS tables that P1P referenced/linked are a good start for how much lime to add. Assuming a meter square, and 20 cm deep, use the center column in each table. If I was adjusting some 40 lb bags of compost I have, they are roughly 40 x 50 × 10 cm, so I would start with 10 % of the recommended lime in the center column of the tables. If you don't have the starting pH of the ericaceous compost, I would assume it is pH of 4 to 5, and use the bottom row of the table. Compost is neither clay nor loam or sand, I would use the texture of the soil you are mixing the compost into, maybe use the loam table if you are not sure.
