@Remy5 I realize this is a little late for this shirt drop, but yes, I personally wear XL in denim/serge Iron Heart shirts, and I size up to XXL in flannels. This is because I hand wash and air dry shirts that are made with dyes that will fade because I have an old school washing machine with no option to stop the spin cycle. With these flannels, I just use the washing machine and tumble dry low. The flannels absolutely shrink some if you do this, as you'd noted. I have three XXL IH flannels now, and they all fit like my other XL shirts.
If you plan to wash and dry the flannel the same as your other shirts, I'd size the same (after double- checking measurements).
