The System Reliability Toolkit derived the MTBF environmental conversion factors from the PIE factors found in MIL-KDBK-217 and the relative component type percentages found in the RIAC database. Calculating the PIE (ML) to PIE (ARW) and PIE (ML) to PIE (GM) ratios for each component type using the PIE factors found in MIL-HDBK-217F, Notice 2 and taking the weighted average according to the percentages listed in Topic in the System Reliability Toolkit yields the following MTBF adjustment factors, which are in agreement with your approach.

ARW to ML: multiply series MTBF by 0.7
GM to ML: multiply series MTBF by 0.3

Note that these conversion factors are based upon the percentage breakdown by component type as listed in the toolkit. If you know that the component type percentage in your design will be significantly different, you may want to adjust your ratios accordingly.