Thin based on observed performance- eg do you feel the brush catching or sticking to the previous patch, or is each new patch blending and laying down?

This is already thin varnish, and you want it as thick as will behave. I usually need to thin on really hot days, or toward the bottom half of the can if I've had to remove a couple of skinned-over layers.

   Did you thin with any specific brand of thinner?  Garage temp will be in 80's so know things will set up fast.  You used the foam brush method for applying?

   I have found that varnish thickens up more quickly if I work from the can. I decant small amounts as needed and keep the can lid tight. 

I experimented with thinning Schooner Varnish but found it went on best without being thinned. I used foam brushes and tried to work very fast. Dip the brush in the varnish. Dab it in about four parallet spots to spread it. And then quickly spread it out the rest of the way with long brush strokes. Keep a sharp eye out for drips and holidays and take care of them immediately.

The good news is that, with four or five coats to apply, you get plenty of chances to practice your technique and learn from your mistakes.  

I use interlux 333 (basically an expensive but very clean version of mineral spirits), using the "plaid" application method, decant to a small cup through a paint filter (never brush from the can), and using a foam brush. A coat on the deck of a kayak will take about 1/3 of a red solo cup, to which I would add at most a cap full of thinner.

I find toward the bottom of each can of varnish, that the surface skins over and the resulting varnish gets slightly thicker each time. I just scoop out the skin, stir, strain, and thin as needed.