Might need a bit more detail here to understand what you are looking for. Usually, people specify the material they want to coat. Doctor blade coating could mean rigid knife coating or flexible knife coating.

RIgid knife coating is fairly tolerant to coating rheology and produces wet coatings something like one half to two thirds ot the gap; however, the wet thickness does depend on rheology (and coating speed). So the typical practice is to allow for adjustment of the blade height to control the wet thickness. Laboratory doctor blades come with either fixed depth or adjustable depth (micrometer). Buying a series of fixed gap doctor blades produces discrete, well controlled thickness samples. Micrometer models are more expensive, but give you the ability to vary thickness continuously. Production knife coaters usually feature adjustable gap and angle. The precision depends on how straight the blade is and how constant the speed is. There are automated lab systems to produce constant speed. But it is hard to get a very uniform coating much under 100 microns (wet). You can go thinner, but the thinner you go, the less uniform it will be.

Flexible blade coating does not have a defined gap, the blade is held against the substrate by pressure and the pressure is used to control thickness. This produced much thinner coatings than rigid knife.

Both methods are well described in Kistler and Schweizer's Liquid Film Coating:

https://www.amazon.com/s?k=kistler+schweizer+liquid+film+coating&crid=19U19OBU52228&sprefix=kistler+schweizer+liquid+film+coating%2Caps%2C74&ref=nb_sb_noss#:~:text=Liquid%20Film%20Coating%3A%20Scientific%20principles%20and%20their%20technological%20implications