Hi! What re some accurate and affordable instruments or indicators to measure flatness?

I am trying to measure the flatness of a steel mold. Although this seems simple, I cannot find an affordable instrument or indicator to do the job. I need it to measure to four decimal places to tell me if it s in spec. The piece I am measuring can be 40x40 inch so that poses another challenge. I ve looked into... show more I am trying to measure the flatness of a steel mold. Although this seems simple, I cannot find an affordable instrument or indicator to do the job.

I need it to measure to four decimal places to tell me if it s in spec. The piece I am measuring can be 40x40 inch so that poses another challenge. I ve looked into CMMs and portable arms, but there must be a less expensive option. Any advice would be appreciated!
Update: Four decimal places in inches**

I already use a granite table, straight edge and a feeler gauge. I am looking for something somewhere in between this and a CMM. I hope this addition helps!
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