The price the tenet pays me leave me with a decent cash flow of about 500 after all payments have be calculated.

Google for a future value calculator.

BP has a similar calculator.

I’m trying to have some idea to calculate for reserve capex needs.

If you or anyone have a good resource or calculator on this, I would greatly appreciate it (hope I didn't break any rules with that statement)  In the past written off all of the HELOC interest, but that was typically what I had used for upgrades and repairs on my primary residence.

View report*This link comes directly from our calculators, based on information input by the member who posted.

I've used the Bigger Pockets calculator do look at the numbers but I think I still need some encouragement to actually go for it.

This is a very "back of the envelope" calculation. 

@Bjorn Ahlblad it is a little hard to calculate the average, which is what got me thinking about posting the question.

I would use a simple depreciation calculation by looking up the average lifetime of your HVAC system and dividing the cost of the system by it.