Short answer is it doesn't translate to grade level work. MAP is not a quantitative reasoning test of the aptitude variety.
It's an achievement test, which does assess substantive knowledge, but not comprehensively. The design of MAP was originally for progress monitoring and universal screening, mainly of at-risk learners. It's pretty good at that, but not necessarily as good at the other end of the curve. The items administered are designed to sample expected skills, not to obtain a comprehensive assessment of mastery of all grade-level standards. So while it's a little better at assessing mastery of grade-level skills than some other instruments, the results still have to be interpreted with a fair amount of caution, especially on the upper end. If you wanted to know if a student had mastered all of the skills and concepts for a certain grade level, you would have to use a criterion-referenced test, not a norm-referenced test, and preferably a curriculum-based end-of-course test (aka, a final exam for the course, taken from the math curriculum in use in the school).
And I'll direct you back to the point that the scaled score is not always derived from assessments of skills at the equivalent grade level. That "ready for algebra" score doesn't necessarily mean she was assessed on pre-algebra skills. Just that she performed as well on the items she was administered (which might have been early elementary basic arithmetic) as a eighth or ninth grader who really was ready for algebra might be expected to perform. Having the same two-digit addition and subtraction skills as a ninth-grader does not equate to being ready for algebra. Or if she took the intermediate level of the test, then it might be that she did as well on late elementary math as an entering high schooler would be expected to do, but she still wouldn't have been assessed on actual pre-algebra skills. So unless she took the grade 6+ version of the test (highly unlikely in a child of this age who has not previously been identified as GT), a score that meets the algebra readiness cutoff tells you very little about her readiness for algebra.
Mainly, it tells you that she has very strong math skills in comparison to her peers, but it doesn't tell you how far above second grade her instructional level is, nor does it tell you if there are gaps that might need to be filled to maximize success in single subject acceleration. You need curriculum-based assessment for that.
