Shopify app - how to apply a single price to a range of values

[loops]

How to assign a value £  to items a custom length within a range of values

e.g  all items between 200-300 cm    price = x

eg enter custom length 220   price is x

enter custom length 280   price is x

all items between 300-400  price y

enter 350  price is y

enter 390  price is y