If using an air drill do the calibration and you will be fine as long as the seed is mixed well and isnt prone to settling out while driving to the field.

Box drills will be difficult from my experience.

Before we added a scale to our box drill we would measure 100’ and run seed thru 1 unit per box, catch it with a coffee can, and weigh it. We typically had it close enough in 2-3 tries. There are calibration widgets out there but have never used one. We didn’t have too much trouble with seed settling out. We usually load for 40-50 acres. With 15 species I wouldn’t worry too much.