D&D 5e Character Sheet Question

Hello, I have a unusual situation and would like to get feedback on how best to implement this. The situation is this. I have a Warlock character who took levels as Rogue. At level 3 they were allowed to take the Arcane Trickster with a slight modification. The modification is such that instead of getting wizard spells, one half of the rogue level will be virtual warlock levels for determining spells per day and level, and for invocations. While this is in no way as it is written in the book it seemed logical and balanced. The question is how would be the best way to get the sheet to reflect virtual levels of a class for the spellcasting calculations? If need be we can track it outside the sheet but it would be nice to keep it in the sheet. Anyone have any ideas? Thanks,

a) Use every 3rd lvl of rogue, not every 2nd. That's how it works in official book. b) So, every 3 rogue lvl just add 1 lvl of warlock in class section. Keep in mind, that you have 1 less d8 HD for every 3 lvls of rogue.

the unintended side effect of that is the proficiency bonus is based on total levels. Since the improvement is only to the spellcasting ability this solution would cause the proficiency bonus to be off at times.

In this case just ignore the spells table and write spell slots based on your houserule - spell slots are all manual. For invocations just keep track of them in the class area. You'll have to calculate how many you have yourself. For DCs just set a custom dc equal to warlock level and rogue level/2. Easy formula.