I believe Psb means pressure at sea level base (or ambient). Using the Workman + GF formula from above:
Workman + GF: M = D * (dM * GF - GF + 1) + (Psb + GF * (M0 - Psb))
Using a depth of 0 for the surface and a GF of 0 the formula reduces to:
M = Psb
Recall from the graphs that a GF of 0 is a TC pressure from a point on the ambient pressure line. A GF of 1 (100%) is a TC pressure from a point on the m-value line. For the ascent to surface, setting Psb to the ambient sea level pressure of 0 and a GFHi of 75%, the Workman GF formula reduces to M = 0.75 * Mo, just as you surmised. Now that we understand the equations let's get back to the question:
Why is Baker's ascent to surface m-value equal to 91.2% of Mo and not 75%?
Workman + GF: M = D * (dM * GF - GF + 1) + (Psb + GF * (M0 - Psb))
Using a depth of 0 for the surface and a GF of 0 the formula reduces to:
M = Psb
Recall from the graphs that a GF of 0 is a TC pressure from a point on the ambient pressure line. A GF of 1 (100%) is a TC pressure from a point on the m-value line. For the ascent to surface, setting Psb to the ambient sea level pressure of 0 and a GFHi of 75%, the Workman GF formula reduces to M = 0.75 * Mo, just as you surmised. Now that we understand the equations let's get back to the question:
Why is Baker's ascent to surface m-value equal to 91.2% of Mo and not 75%?