At what approximate block height/date will the percentage of mined pre-tail Monero emission equal the same percentage percentage as Bitcoin?
This will happen at approximately February 11, 2017. At this date approximately 75.72% of the pre-tail Monero will be mined. In the spreadsheet it is marked as "Total Emission Crossover".
Is there a spreadsheet or calculator that can easily help answer identify the percentage of pre-tail coin supply that will be mined as of a specific blockheight/date?
Yes there is.
For clarity, I have used this spreadsheet to answer your first question as well. Credits for the spreadsheet go to core-team member luigi1111.