Hi @microbiomeAnalyst,
Your calculations look right to me.
Assuming you are planning on using dada2, the required overlap you want to keep is 20nt+ natural variation in your target. For example if your expected target of 440bp can also be 430 bps, then you would need 20nt+10nt minimum overlap for proper merging, so your 56bp sounds pretty safe to me.
Not unless you were including these in your original calculations which I'm guessing you weren't. The trimming and truncating parameters refer to positions on your reads so your trimming (at 5' end) wouldn't change the position of your truncating (3' end) so you don't need to adjust your overlap calculation. But as you indicated, you should make sure your primers are removed prior to denoising.