For glassing, I calculated about 250gr/per m2 surface
(what I've seen when glassing my MIG15:
Note that with balsa, I expect more epoxy to be pulled into the wood. I would say about 25% more.
This is with 25gr/m2 gloth and 2 layers of epoxy.
So if you have a 20dm2 wingarea, the added weight would be = 20/100 * 2 * 250 = 100gr.
What is the added weight for what you mentionned "tissue, paint and clear varnish" ?