According to AMCAS data the exact balance of MCAT:GPA varies with a pretty consistent tilt in the direction of the MCAT. An actual weight table is in the med school spreadsheet or can be created using the AMCAS MCAT vs. GPA % Acceptance grid. The LizzyM formula is a good approximation for this as long as the GPA is ~3.7-3.9 and the MCAT is ~28-32. Outside those ranges, the MCAT gains importance quickly as it rises until about 35, where it begins to lose relative value per point (or loses importance as it falls below ~25). Simultaneously, the relative per point value of the GPA increases consistently throughout the entire range such that, per point, a 3.9 holds more weight than a 3.7. (Of course, if one's GPA & MCAT both rise, the relative proportionality of the two both rise and cancel each other out.) The LizzyM, therefore theoretically departs most from expected values at a high MCAT as the MCAT loses value/point at extremely high values (unlike high GPAs -- ultra-high MCAT scores are simply seen as not showing much beyond the fact that the person is an extremely good test-taker). The LizzyM 1:1 assumed ratio actually breaks down most when the MCAT and GPA are most different. A high GPA with a low MCAT gives weight in the GPA direction as long as the GPA is strong (3.5 -- they equal out at about 3.5 if the MCAT is 25).
The reality, however, is that these changes throughout the range make little difference in most cases. As a general rule, the MCAT is weighted slightly more strongly than the GPA on average across MD schools according to AMCAS.