Simple gathered dress for a child

[Ellabella]

I want to make my GD a simple gathered dress similar to this https://tuclothing.sainsburys.co.uk/product/tuc143233750
I have some awesome Wednesday Addams fabric she will adore.
I have a basic long sleeved tee shirt pattern that fits well so intend to just add a gathered skirt.
Is there any magic ratio for calculating the width of the gathered part or am I overthinking this as usual?

[HenriettaMaria]

I would say, it depends: the weight of the fabric and how much fullness you want will govern things.  I would do a measured length of 5mm running stitch in a similar fabric and draw it up until you like it.  Then measure the length of the gathered section and compare with the original length to give a multiple for the width you need when cutting out.

[Greybird]

The one in the picture looks to be about 2:1. I don't think I'd do it any less than that and it wouldn't need to be any more than 3:1.

[Starryfish]

I agree with Greybird, 2:1 would be about right. If you have too much gathered jersey it would be bulky and may drag the bodice down.

[Ellabella]

2:1 it is then. I’ll let you know how I get on. 

[BrendaP]

Looking at the picture I would say it's only about 1.5:1.  There isn't much much flare in that dress.

[Surest1tch]

Looking at the pic it doesn't look like there's much width around the hem area, unless you're going to make it quite short I'd be inclined to do double width and tack it to the bodice before you machine it and see how much stride room she's got, if it's too much you can always take some away from the sides without too much frustration for you. That's all saying you don't want to make it as a surprise for her