Can anyone help with this – got to enlarge a plan for someone and maths has never been my strong point!

2500%

Hmmm – my customer must have that wrong. Enlarging an A4 plan by 2500% would create one massive sheet.

What would 1:500 to 1:200 be percentage wise?

1:500 is 0.002 (1/500)
1:20 is 0.05 (1/20)
To get the ratio 0.05/0.002 is 25 so a 2500% increase as jam bo says

You’ll not scale from A4 to a printable size without cropping it.

Your second suggestion is only a factor of 10 different, so 250% or 2.5x

The percentage scales between paper sizes are on a table down this page
https://www.cl.cam.ac.uk/~mgk25/iso-paper.html

or google it

My thought was 2500%

imho percentages are horrible for such calculations. The scale factor is much clearer as Jon Taylor shows. ‘Percentage increase’ is also a little vague; the linear scale factor is 25, but the area scale factor is 625. So, depending on what you’re trying to calculate one might be more appropriate than the other.

Apologies if I have misunderstood.