Is calculating the diameter of glass as four feet (30 cm square per foot), as it is purchased, considered impermissible (haram) or permissible (halal), or is the geometric calculation (2.1 feet) the just approach to avoid falling into the impermissible (haram)?

It is permissible to draw on glass as long as the drawings are not of animate beings. Honesty in selling is obligatory, and cheating or excessive fraud is prohibited. If the glass circle can be measured by two different estimations (4 feet and 2.1 feet), it is permissible to sell it according to one of the estimations, provided that both the seller and the buyer are aware of the truth, or if the commercial custom is consistently applied. If the custom dictates the engineering estimation (2.1 feet), then selling it as 4 feet is forbidden fraud unless the buyer is explicitly informed of that.