Price according to variable typed by the customer

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I am looking to provide a service where I "CUT-TO-SIZE", in which I would like the customer to type, or select from the dropdown, the closest next round number twice...  


i.e. Client wants to cut to 2.5 x 4.75. Closest round numbers would be 3 x 5, which is what they would have to type/select and this would be calculated through some matric to price for checkout. 


Any thoughts?