When you state a % you are being pragmatical, but if you are rasing a convertible note valuation might not be so clear you will then be defining a cap and a discount. Silicon Valley VC's want to understand more what your burn rate is for the next "x" months and what would be your next funding or goal event. So in that sense you might find that your ask will be for the total amount needed to cover full expenses for a team of 5 for 18 months at $10-15K in a city like San Francisco till you reach your next milestone or funding event.

Happy to talk more strategy on a call