At the risk of sounding all zen.
I don't paint a cross,that's just the space left over when I've finished painting.

Because it's easier to paint convex curves rather than concave.Thats all I paint.
The cross is a series of black concave curves on a white field.but a simpler way to look at it is to see a series of white convex curves on a black field because that's the easiest to paint .
I mark out a square in a thin line of white paint and then the diagonals(on a black field) then I add a thin curve of white to the diagonal and repeat on each diagonal 'limb'
I should point out that you'll find it easier to paint a curve in one direction ie left to right,or right to left.So only work with the one curve and turn the model around to do each curve (not your hand) go right around the 'cross ' on one side only so you end up with a series of 'D's forming a propeller,the reason for this is it's easier to keep a series of single curves the same size.
Once you're happy with the sizes turn the figure around and finish the other curves remembering to work with the direction you find easiest,and turn the figure not your hand for each curve.
It sounds long winded but it's just working with the one curve you do well,and using it to achieve the ones you don't.Because a black concave curve on a white field is also a white convex curve on a black field when you take a second look.
Hope that helps (if not then it's 1/72nd biplane transfers for you my lad

)
Mark.