I know this topic has been discussed a lot so please be patient with me

I am designing a rectangular internal pressure vessel using ASME BPV Sect8 Div1 Appendix 13, specifically sketch 6 with chamfered corners and partial reinforcement. I set up a spreadsheet and was pretty happy with the stress figures i got. However to check i set the spreadsheet up correctly i then set my dimensions so the width equalled the height and the type and length of reinforcement was the same (i.e. L3=L4, L2=L1 and L11=L21). With this set-up i was expecting stresses at corresponding points to be equal due to the symetry (i.e. stress @ A = stress @ H, stress @ B = stress @ G, etc.). Most of my points match up except for M and N

When i look into the equations further i found my value of 'thetaM' exceeds the value of 'theta1' (theoretically not possible??), and 'betaN' doesn't seem to correspond in any way

I've doubled checked my spreadsheet 100 times, set up a completely new spreadsheet from scratch, and even had a colleague set up his own spreadsheet with no input from me. We both keep getting the same issue.

Has anyone else come across similar issues? Can anyone point me in the direction of a solution?