Do you have a manual for the cutter? Mine is a Bush Hog BH26 and it has the same set up. You have the angle of the top link about right. FWIW, according to the manual, you set the front of the cutter to the desired height, then adjust the top link so that the rear of the cutter is an inch or so higher. When all is said and done, when you raise the tractor lift, the front of the cutter should raise 2" before the rear of the cutter comes off the ground. Again, this is from the BH26 manual. Let me know if this is the model you have and you need a manual.

My $.02 on the chain deal. The argument I was always given against using a chain was if the rear of the cutter got too high, the angle of the PTO shaft could damage the tractor PTO. This would come into play more if backing into places to cut ditches and such. Like Daniel, I've used a chain top link a lot in cutting plots and pasture. We've also disconnected the top link to mow, letting the cutter float with the lower links, then connect the top link for transport. We've also used a ratchet strap for a top link in a pinch. This may not be recommended, but a lot of folks still use the chain.