I recommend you make and maintain your starter at 100% HYDRATION. This means the same weight of water to flour. It creates a starter that is easy to handle, and once active and vigorous, less likely to suffer problems with storage in the refrigerator (up to a week). It results in a less sour tasting bread. Keeping this proportion constant helps minimize variables.
Dough for prosphoro is very different to most bread doughs. It needs to have a much lower hydration, for better stamping and to lessen the development of large air cells.
For every 100 grams of flour we use 50mls by weight, of water.
As an simple starting "rule", use between 5% to10% of the weight of flour as the weight of the starter. So, for a prosphoro made with 800 grams of flour, we use 40 - 80 grams of starter.
For every 500 grams of flour we use 3.5 grams of fine sea salt or cooking salt. (table salt has other ingredients added)
SIZE OF BAKING PAN - PROPORTIONS AGAIN
Many beautiful prosphora are made without a baking pan. However we start with pans, the most straightforward technique and will cover some of the other traditions in my blogs.
Using incorrect proportions will cause problems. It will affect the height of the prosphoro and invite difficulties during the shaping, stamping and the baking stages.
It is best to obtain pans with tall sides. They are easier to handle with less chance of causing damage to the surface of the prosphoro while it is rising.
The stamp area must be at least a centimeter (preferably more) away from the inside edge of the pan. If the impression is too near the pan, then as the dough rises and expands, the outermost parts of the impression of the seal may be lost, dragged down the side and against the pan.