No
You are calculating the average cost per kWh, when you are probably interested in the marginal cost per kWh (weight averaged across any price tiers you may cross.)
Here is the problem: Say you want to know how much it costs to fuel your EV, and you know how many kWh the EV consumes each month
Your electric bill is stable month to month (just an exercise to point out the problem with your method)
Can you tell me what your future bill will be with your method ?
The answer is a big No. How much you will be off varies by the specifics of your utility billing structure and your baseline consumption, but your estimate can easily be off by 30% for the EV portion. This thread has already posted the correct way to calculate marginal kWh cost.