Tartine's Lemon Meringue Cake in Sheet Form

My daughter's 18th birthday party is this weekend. She wants the Lemon Meringue Cake from Tartine's book, but wants it in the shape of 1 and 8. This all seems fine but the recipe/directions are for round cakes.

My thought is to use a sheet pan (12x17) and cut into quarters to do the layers and use to shape the "1". Then do two small round cakes to shape as the "8".

The recipe/directions are for 10x3 inch springform. The volume of the spring form is ~235 and the sheet pan volume is approx. 204. Question: should I use a proportional amount of batter (i.e. 204/235), approximately, to fill the sheet pan? I will cut the sheet cake into four equal pieces and layer them to make the four layers. I want to make sure I end up with a sufficiently "high" cake in the end, but don't want to over fill it ending up with a mess. Tho I can always place something under to catch run over, this outcome remains a bit undesired.

Any input is appreciated.