Providing the essay, letter of rec, academics, and extra curriculars are substantial, is a 1390 (710 RW & 680 M) considered weak for a prospective Dartmouth applicant? Or is it average?
From the Dartmouth website:
SAT AND ACT TESTING FIGURES FOR THE CLASS OF 2020
SAT
SAT CRITICAL READING
Mean: 717
Mid 50% range: 670 - 780
SAT MATH
Mean: 723
Mid 50% range: 680 - 780
SAT WRITING
Mean: 722
Mid 50% range: 670 - 790
You are a little below the average, but remember, the mid 50% also means that half of their admitted students get scores below the average (or scores similar to yours) These students must be very strong in all other areas to compensate for their slightly lower test scores. I would give it a shot, you may be surprised! Best of luck 
Your SAT’s are low for Dartmouth. Keep in mind that the averages include scores for “hooked” applicants as well (ex. recruited athletes, legacy etc.).
It is fine to put in some reach applications but be sure to create a well balanced list that also includes match and safety schools that paper affordable and that you would be happy to attend. Thus far every post you have on CC is for a school that would be a huge reach given your SAT’s (ex. Yale, MIT, Dartmouth). I’d suggest that you spend some time focusing on match/safety schools.
@grapevine88 doesn’t it mean that 25% are below the score range and the other 25% are above the range?
Yes. and it also takes in consideration students who who are attending, not simply students who were admitted (remember, everyone who was admitted does not attend).