It’s not about SCM replacement, it’s about cement content. A mix with 500 pcy of cement + 50 pcy SCM vs a mix with 500 pcy cement + 100 pcy SCM will have very similar impacts. The second one will likely be stronger, but SCM is still an indirect metric. If you’re going to use prescriptive specifications, I would recommend maximum cement content and refer to the NRMCA regional benchmark as a baseline.
Like @ajauer said, PLC/GUL is also a great way for easy reductions. You can also be more cognizant of specifying curing time (i.e. maybe you can go with 56-day strength for foundations). Next step up is going with performance-based requirements by specifying maximum GWP as demonstrated by a product-specific EPD. https://www.portlandoregon.gov/brfs/article/731696