Not too far from you in central MN, My seems to be proven method for decent planning and speed planning is. 500 FPM descent with a mile to spare before pattern. I reduce MP at 1" per min to end up at 15 inches at pattern entry. My speed is right on and never struggle to slow down also I don't need to worry about rapid cooling of the cylinders. If I am doing a long final or flying an approach I may need to add power to maintain speed typically 17 inches to maintain 100 kts with gear and flaps out. I fly downwind at 100, base at 90, final at 80 till short final at 71. Also I fly an 87 J model 205. Also I agree with others find a CFI with Mooney experiance.