My approach to claiming tax deductions will be to record actual kWh used in the year (which the Model 3 will tell you from the Trip A measure) and apply the normal grid rate (in my case, around 40c per kWh) to work out the notional cost of electricity. Then claim x% for business use.
About 75% of my charging is from solar, plus a tiny bit from supercharge, some free, some from grid. It is too hard to work out an 'actual' cost, given all these sources.
Therefore, the tax office should accept the reasonable basis to calculate your deduction as I've outlined. I am a registered tax agent, Chartered Accountant etc - but give you this suggestion with all the usual disclaimers of not knowing your individual circumstances (eg are you in a company, is FBT in the mix etc).
If somehow you accidentally reset the Trip A, then using an average efficiency rate along the lines outlined by
@EcoCloudIT would also be a reasonable approach to calculating your notional cost.