Foundry specs are typically +/- 50mV 3 sigma. This applies to across the wafer and wafer to wafer so no points should be worse than +/- 50mV from nominal. This applies to all transistor sizes but typically only 10um x 10um, 10um x Lmin and Wminx10um is checked. Wmin x Lmin is probably not checked or if it is, the spec may be wider..
The threshold voltage is calculated by ramping Vgs from 0 to 1.8V with Vds = 50mV (or sometimes 100mV). The maximum slope of Ids vs Vgs is calculated and extrapolated back to zero Ids. This zero value IDS is the threshold voltage (Vt0).