Cutting new parts. Inside line/Outside line ?????

[buglecall]

I am new to the hobby with several builds/attempts done.
With the guillows kits i find the parts annoying from kit to kit and indeed sheet to sheet! My last kit had a fantastic sheet of formers and ribs and the other sheet horrid crushed and with really uneven edges.
So i going to attempt cutting my own.I have noticed on previous kits the parts never truelly match the plan.
So here is my question,i am building the Spitfire "403" kit. When i cut new parts do i cut to inside line or outside? I realise if i cut to outside i can fettle if needed but i would rather get it right first go.
But why do the parts seldom match the the plan? Is the plan off or the parts?

[SteveM]

Photo copy the parts templates on the plans and use those as your guide for cutting new parts. Cut a little outside the line then sneak on the line by sanding carefully. The plans are what you want to go off of and they frequently do not match the die cut parts. I would imagine that the mismatch is cause by the difficulty and cost in making the dies a perfect match to the plans. In theory the laser cut kits would have parts that are a perfect match to the plans, but I've not built one from Guillow's yet.