The level you should be at is determined by how fast you recover, typically you would like to recover from the max sustained heart rate about 25-30 beats per minute in 30 seconds.

So just for example if you hold your rate at 175 bpm you would like to recover to 145-150 bpm within 30 seconds of stepping off the machine. so if your not recovering fast enough go back down to a level were you do recover within those ranges and you will eventually be recovering faster so then move your sustained heart rate up.

And if I may suggest move up in 5 bpm increments and also train 20-30 mins 4-5 times a week for best results. you will be surprised how fast your heart gets stronger it won't take long to move your bpm up.