Been using “backer rod” since about 2016 with no adverse effects. It’s cheap, effective and works well for me. The “toxicity “ of a fraction of a grain by weight is probably overshadowed by many other secondary sources of exposure that comes with a day at the range IMO. I have found that the exact length of cut backer rod is less critical so long it’s above the minimum length of your loading. Measure from powder charge to case mouth, and cut to that length. For 45 Colt, the seating depth of your bullet will supply plenty of compression with APP, BlackMz, T7etc… Add about 1/8” length for real black.