Hi Al - I don't know if you read the Des Moines Kubb blog, but I wrote a piece on efficiency that you might find interesting:
Your estimate of 1,5 to 2,0 field kubbs per hit is right on - in the data we've got the 'tournament level' average is right around 1,6.
But that is a very general number, and is only useful over large numbers of rounds. In the part 2 post above there is an image (that I am having trouble posting here) that details the average 'Baton Value' for any given number of field kubbs - I'll copy the meat of it below, and present it as "X : Y,yy" where X is the number of field kubbs thrown, and Y,yy is the average number of batons required to topple all of them.
1 : 1,53
2 : 2,36
3 : 3,04
4 : 3,63
5 : 4,18
6 : 4,68
7 : 5,15
8 : 5,59
9 : 6,02
10 : 6,43
Base Kubbs cost about 2,85 and Kings cost about 1,20.
This is a pretty good yardstick to measure yourself against, and is a great practice tool. My advice would be to focus on throwing groups of 5's & 6's as they seem to show up most often in the game data we have. If you can consistently clear 5 with 3,0 to 3,5 and 6 with 3,5 to 4,0 then you know that you are playing very efficient kubb and are giving yourself better-than-average opportunities to make progress on the base line.