Where the default values come from
The defaults are the planning allowances in the TIA-568.3-D cabling standard (the 2016 edition, replaced by TIA-568.3-E in 2022): up to 0.75 dB per mated connector pair, 0.3 dB per splice, and fibre attenuation of 3.0 dB/km at 850 nm for OM3, OM4 and OM5 multimode (3.5 dB/km for OM1 and OM2), 1.5 dB/km at 1300 nm, and 1.0 or 0.5 dB/km for single-mode inside and outside plant. Real components are usually better than these limits. Replace them with the values on your cable and connector data sheets, and take the allowed channel loss from your transceiver's data sheet.
For MPO links, count every MPO-to-MPO mating and every cassette as a connector pair. Low-loss MPO connectors are specified tighter than the standard's allowance, which is why many high-speed links rely on them. The loss budget guide walks through a full example.