HELOC payback calculations

[Christopher Davis]

Very first investment property. Let's say I'm buying a duplex for 350k. I might stay a little more conservative for a first property, but let's use this as a model for this question. 

I have 40k in cash, reserving 10k for rehab. So 30k cash.

Requires 25% down, so 87,500. 87,500-30k = 57,500 for the down payment. 

I could do a HELOC for this. Plus the loan. This seems risky.

If I do a HELOC for the 57,500 down payment, how do I calculate a sensible payment plan for that? It's not a standard amortized loan. It's more like a credit card at 4-6%. I can pay back as much or as little as I want per month.

I can calculate the principle loan, but how do I manage the HELOC pay back?

Is this an advisable strategy? Alternatives?

Thank you!