I add some example taken from the web, unfortunately they are not mine:
1) Cremona - Milano VALORE DICHIARATO. The rate comes from the weight and the value:
WEIGHT : 791 Napoleoni oro X 6.43 grams= 5086 grams = 9 pfund and more as written on the cover. The rate on the II distance (5- 10 leghe tedesche) was Lire 1.00
VALUE: the rate between 18300 - 18600 lire on the II distance was Lire 6.20
+ 50 centesimi fixed fee Total Lire 7.70 prepaied by the sender.
2) Portobrief Montagnana - Mantova VALORE DICHIARATO. The rate comes from the weight and the value:
WEIGHT: 100 Napoleoni oro X 6.43 grams = 643 grams= 1 pfund and more as written on the cover. The rate on the II distance (5- 10 leghe tedesche) was 8 NKR
VALUE: the rate till 800 florin was 32 NKR
+ 15 NKR fixed fee Total 55 NKR + 5 NKR for stamp duty prepaied by the sender
I hope my calculations are right
Francesco