From a GAAP perspective the traditional technique was to debit Cost of Sales for warranty reserve costs upon sales recognition. That, however, is a non-cash charge. The charges to warranty reserve tend to be multi-modal with peaks in first month of sale, last months of warranty, and one or two other items that vary depending on model-specific characteristics. That is a tradition ICE picture, but very little alters with BEV's in general or Tesla specifically. An added complexity is that mew model or major modification to existing model produces, invariably, a higher early mode, frequently for several months or more.
In forecasting TSLA I would make a smooth assumption for cash flow perspective, and follow a simple percentage of average selling price for calculations of GM. Frankly, we haven't adequate information to refine this further and anyway, given the assumptions that must anyway be made, more refinement will probably increase potential errors rather than reduce them.
FWIW, conservative manufacturers over-reserve for new models and new markets, gradually reducing them when actual results add to predictability. Of course, when major recalls or other defects appear, that can go the other way, as it sometimes does. Mostly, when the initial assumption prove too optimistic the vehicle either is withdrawn for sale or is redesigned.
