Here are the percentage errors for the numerical relationships given in Richard Hoagland's book The Monuments of Mars. I am using equality signs for these approximations because Richard Hoagland does that in his book and on his website. Some of them are actual equalities (indicated by a percentage error of .0%), resulting directly from the fact that A = 60^o and B = 120^o.

C/A =    \/2                          (.5%)
B/D   =   \/3                          (.17%)
C/F   =   \/3                          (.7%)
A/D =    e/pi                      (.08%)
C/D   =   e/\/5                     (1.1%)
A/F   =    e/\/5                     (.5%)
H/G =    e/\/5                     (.9%)
C/B   =    \/5/pi                   (.13%)
D/F   =   pi/\/5                    (.4%)

Tan(A) =    \/3                  (.0%)
Tan(B)   =    \/3                  (.0%)
Sin(A)    =   e/pi                (.09%)
Sin(B)     =   e/pi               (.09%)
Tan(F)    = pi/e                (1.6%)
Cos(E)    =   \/5/e              (.05%)
Sin(G)     =   \/5/pi             (.5%)
Sin(C)     =    1                    (.33%)
Tan(G)   =    1                    (.35%)
Tan(I)     = -2e                 (.22%)

Rad(A)    = pi/3                 (.0%)
Rad(B)    =   2pi/3             (.0%)
Rad(D)   =    e/\/5              (.36%)
Rad(F)    =   e/pi                (.05%)

Tan(40.87^o) = e/pi (.006%)