The other problem is confusion

I have never made a wine that was made up to the full gallon at the start. If I had, it was be easy to take a reading. I ferment in 1 gallon demijohns and always leave a head space for the first week or so until the PF has subsided, then I top up, fit an airlock, and ferment out.
The other method is fermenting on the pulp. Bung all the fruit in a bucket, (sometimes with sugar, sometimes without), leave to ferment for a week, strain off into demijohn (adding sugar if not done already) and top up with water.
When is the best time to take a reading using the above methods? I guess, using the first method, an accurate reading could be achieved if it was made up to the gallon at the start, then keep some of the liquid in a bottle which could be used for topping up the demijohn.
What about the second method? The yeast is already working on the fruit sugars so a reading will be useless a week down the line when it all gets strained off into a demijohn and topped up.
Maybe I should carry on as normal, being blissfully unaware and hoping for the best?

Beer I get: make it up to X gallons, take reading, it is what it is
