QFE is not used in the US. If you reference FAR 91.121 it will answer all of your questions about altimeter settings in US airspace. Basically you will be using QNH your whole time below 18,000 ft.
Sec. 91.121
Altimeter settings.
(a) Each person operating an aircraft shall maintain the cruising altitude or flight level of that aircraft, as the case may be, by reference to an altimeter that is set, when operating--
(1) Below 18,000 feet MSL, to--
(i) The current reported altimeter setting of a station along the route and within 100 nautical miles of the aircraft;
(ii) If there is no station within the area prescribed in paragraph (a)(1)(i) of this section, the current reported altimeter setting of an appropriate available station; or
(iii) In the case of an aircraft not equipped with a radio, the elevation of the departure airport or an appropriate altimeter setting available before departure; or
(2) At or above 18,000 feet MSL, to 29.92'' Hg.
This information will be provided to you by ATC if you are IFR or using radar services. Otherwise you will have to tune in an ATIS or AWOS along your route.