As ninthace posted a simple bit of trigonometry gets you the answer.

It's only a simple bit of trigonometry you know trigonometry! I just looked up the meaning of the word:

Trigonometry (from Greek trigōnon, "triangle" and metron, "measure") is a branch of mathematics that studies relationships between side lengths and angles of triangles.

So had I known what trigonometry was (other than the bit of maths dealing with Sine, Cosine and Tangent, mysterious functions I've copied and pasted - my understanding of trigonometry) I still wouldn't have known I needed trigonometry without knowing it.

It's only easy once you know! My corrected sum based on Alan's lesson about mils is also simple ((6283 /360) * 2) and was only 6 millimetres out. Don't think I did too badly with my limited knowledge of maths.

Crikey I learn a lot more than just walking here.

I get that there will be other errors introduced by walking as you all point out but if I've broken the nav rule of my book and don't have an attack point within 500 metres, all the more reason to do it properly and correct for it - it's an additional error I can do without. Will it matter in practice? I'll take the forum at it's word it won't.