There could be an error. Buy all parts before you start. Verify measures between the bought parts, the design and your buggy. Adjust the measures if needed. Take this drawing with you when you go shopping.
Good advice is always cheaper.
I guess the rear wheel at 400 mm behind the rear-axle is enough to allow the use of the Spring in the Back . If you are one of those who sit up straight on the buggy then maybe you could put the rear wheel at 250 mm from the rear-axle. I used an angle of 50 degrees, instead of 45, to gain approx. 3.5 cm extra ground clearance. You will need something extra because the rear tire carries now twice as much weight.