Tom,

My L39 balances out between 85 and 90mm back of LE. Like you said, hard to say with finger tips...
A friend of mine who is a way better pilot than I am, felt it could be balanced a little more aft. He flew it several times and still feels this way. Last time I checked, my L39 was 3.7 kgs dry. Probably a little more now with the oleos I just fitted. No nose ballast was required.
Let me know how CG check turns out on your reaction 54.

Yann