An answer offered from another forum:

It can be derived as a function of the difference in gravity

Pre boil Volume  = Post Boil Volume * (Post Gravity Points / Pre Gravity Point)

In this instance, pre gravity is 1.021, post is 1.0.30. Post boil volume is 23.7 litres

Pre Boil Volume = 23.7 * (30 / 21)

Pre Boil Volume = 33.9 litres

An answer offered from another forum:

It can be derived as a function of the difference in gravity

Pre boil Volume  = Post Boil Volume * (Post Gravity Points / Pre Gravity Point)

In this instance, pre gravity is 1.021, post is 1.030. Post boil volume is 23.7 litres

Pre Boil Volume = 23.7 * (30 / 21)

Pre Boil Volume = 33.9 litres

An answer offered from another forum:

In can be derived as a function of the difference in gravity

Pre boil Volume  = Post Boil Volume * (Post Gravity Points / Pre Gravity Point)

In this instance, pre gravity is 1.021, post is 1.0.30. Post boil volume is 23.7 litres

Pre Boil Volume = 23.7 * (30 / 21)

Pre Boil Volume = 33.9 litres

An answer offered from another forum:

It can be derived as a function of the difference in gravity

Pre boil Volume  = Post Boil Volume * (Post Gravity Points / Pre Gravity Point)

In this instance, pre gravity is 1.021, post is 1.0.30. Post boil volume is 23.7 litres

Pre Boil Volume = 23.7 * (30 / 21)

Pre Boil Volume = 33.9 litres