In API formula, you have to subtract from Pi=2.5 [psig] =69.2 [in H20] the equivalent effect of roof weight which is in your case
0.245 DLR/D^2=0.245*4794/27^2=1.61 [in H20]
In the previous editions of API 650 that term to be subtracted was "8*th", in your case it would be 8*0.1875=1.5 [in H2O].
I would conclude that it is nothing wrong with your "DLR" value.

Of course you shall obtain a bigger value as "A", just follow the correct API formula an pay attention to the units required.
Maybe the mistake is rather the input of Pi as 2.5 inch of water.