Look up the data for the bullet you are using. Use that COL.
Which bullets have a COAL designated for them? I must have skipped that page in the reloading manual.
Or, what I do is read the data several bullet makers and strike an average mid range charge.
For the OP, if you have data for a particular bullet in a particular cartridge, I'd stick with that. Different bullets in the same caliber and weight can vary a lot in how much pressure they generate using the same powder and charge.
