Mine fluctuates constantly, never more then 3-4 months at the same amount.I'm not aware of HELOCs with fixed rates, i thought they where all a variable interest rate product.Here is a post to exactly how I use my LOChttp://www.biggerpockets.com/forums/48/topics/100685-here-is-how-i-do-it-please-critique

Another income point is that loans are done when sales are slow in refinancing but the mortgage market fluctuates as well.You may excel quicker as a Realtor, if you're good, getting into higher incomes as a mortgage originator take time as you're more of an captured employee.  

Gas is the big one that fluctuates in Michigan so I calculated the average.

Can use as a "guestimate", I would set aside 15% of that total for the unforeseen and fluctuations of accual cost vs the guidelines.

In reality, that percentage fluctuates a bit, but that's the rule of thumb.

Additionally, vacation properties are rented daily/weekly and can be very susceptible economic fluctuations.    

Sure, CAP rates are going to fluctuate with interest rates, they have too. 

I do have a renter in there now for 16 more months, and am more than willing to wait out the storm of fluctuations with home prices.

Obviously this experience helped me to understand why people struggle here all the time with identifying high volatility and fluctuation on prices. 

I personally would hire a quality and experienced project manager, have their pay fluctuate in line with how well the project goes so they are personally invested in the project and the better you do, the better he or she does financially.