Depending on your specific circumstances, your water may be colder than that...it's generally not as cold there as here, but I've seen incoming water temps in my NH condo at 33-degrees and I'm near sea level fairly close to Boston. Depending on the cold water temp, you will use various proportions of hot/cold, and keep in mind that 120-degrees at the outlet may or may not be 120-degrees when it hits the supply valve of your shower.
Some people love their tankless, some don't. You can do the math yourself...for rough calculations, a gallon of water is 8#, and it takes one BTU to raise one pound one degree. Your tankless is BTU/hour, so divide by 60 to get a one-minute output.