It is an imperfect science.

Your question has about 1000 answers and this is why..... The shape of the weight, the diameter and type of line, underwater current, surface conditions, type of rod and type of lure or spinner all effect the depth you want to reach.

In my experience, If I want to reach 15 ft down for steelhead with 1oz slender trolling weight going 3mph, I need 100 ft of line out.

Your scenario is .8-1.5 mph. 1 oz weight should be at 15 ft at 45-55 ft of line. I drop 1 ft per 3 ft of line let out at 1mph.

Use good line. I think braid is best to optimize depth. Use line counters and play a bit and once to start to catch fish, go back to that number.