When making bread, I've always tended to work by eye - starting off with a certain amount of water and then adding flour until things felt 'right', which generally gave me proportions of about 1 part water to 2 parts flour. However, one of the things I learned as a result of my sourdough quest was that wet dough is, at least up to a point, better - enabling you to produce tastier bread with a better texture.
The technical term for the proportion of water in bread dough is 'hydration', which is calculated by dividing the amount of water by the amount of flour and expressing the result as a percentage. For example, if you make a 1 kg loaf using 400g of water and 600g of flour, your hydration is 400/600 = 2/3 = 0.67 = 67%.
This may all sound needlessly 'techie', but you will soon find that expressing your recipe as a hydration percentage gives you a very good indicator of what kind of dough you will be working with. And if you want to scale your volumes up or down, or change some of the quantities then identifying how this effects the hydration percentage is essential to ensuring that you still produce the right kind of dough.
As we can see from the example above, calcualting the hydration percentage when making bread from scratch (using water, flour and live or dried yeast) is simple enough - just divide the water by the flour and express the result as a percentage. However, when making sourdough things become more complicated. This is because you are adding flour and water to your existing starter or ferment and so in order to calculate the overall hydration percentage you need to take into account not just your added flour and water but also the amount of ferment used and the proportions of flour and water it contains.
At this point, the maths becomes a little complicated and the easiest solution is to use a hydration calculator. There are several of these available online, but I decided to have a crack at creating my own from scratch.
The calculator divides into two stages: starter and dough, for each of which it allows you to work out the hydration percentage of any given combination of water, flour and starter, and also to perform a reverse calculation so that if you want to a specified volume of dough (e.g., 2kg) at a given hydration level (e.g., 67%), containing a given amount of starter (e.g. 400g), you can automatically calculate how much flour and water you need to use to achieve this.