IHT 400 No will

[alittleunsure]

I have been trying to support my dad and complete this form to enable Grant of Probate/ Letter of Administration but some of it is making me feel like I am wading through treacle. I appreciate that things would have been much easier if they had a Will. The IHT 404 and subsequent filling in of the IHT 400 is tricky. If anyone could give me any advice, I would be very grateful.

Exemptions and reliefs for the jointly owned assets shown in box 1
As the house was in joint names and will be passed to dad, I have included my mum's share so Net total of jointly owned assets (box 3 minus box 4) is 0 and I copied this amount to form IHT400, box 49.
She didn't own anything solely for 51 - Deceased’s residence (except farmhouses and jointly owned houses). However, question 59 asks for box 49 plus 51, which is 0 and the editable PDF won't accept or retain the 0. I can only imagine I have done something very wrong.

I hope this makes sense.

[maths]

Are you sure you should be filing IHT 400?