I think your calculations might be a little backwards in places.
Erid (40 Eridani A b) has an orbital period of 42.245 Earth days. That's how long its year is.
It has a solar day period of 18397.44 Earth seconds (5.1104 Earth hours).
We can find the length of an Eridian year in Eridian days with this formula:
Eridian year = (42.245 * 86400) / 18397.44
↳ Eridian year = 3649968 / 18397.44
↳ Eridian year ≈ 198.4 Eridian days (rounded to one decimal place)
Eridians have an average lifespan of 689 Earth years. To convert this into Eridian years, we can use a similar formula:
Average lifespan = (689 * 365.5) / 42.245
↳ Average lifespan = 251830 / 42.245
↳ Average lifespan ≈ 5961.2 Eridian years
Rocky has been mated to Adrian for 186.3 Earth years. Using the above math, this gives us a number similar to your first one: ~1611.85 Eridian years.
Here's a reference chart.
Length of an Eridian Year
Earth units (Base 10): 42.245 days
Earth units (Base 6): ~110.125 days
Earth units and Eridian numerals (Base 6): IIℓ.IV∀ days
Eridian units (Base 10): 198.4 days.
Eridian units (Base 6): 530.224 days.
Eridian units and numerals (Base 6): ∀λℓ.VV+ days.
Earth units (Base 10): 18397.44 seconds
Earth units (Base 6): ~221101.24 seconds
Earth units and Eridian numerals (Base 6): ~VVIIℓI.V+ seconds
Eridian units (Base 10): ~7776 seconds (an Eridian second is ~2.366 Earth seconds)
Eridian units (Base 6): ~10000 seconds
Eridian units and numerals (Base 6): ~Iℓℓℓℓℓ seconds
Earth units (Base 10): 689 years
Earth units (Base 6): 3105 years
Earth units and Eridian numerals (Base 6): λIℓ∀ years
Eridan units (Base 10): 5961.2 years
Eridian units (Base 6): 43333.111... years
Eridian units and numerals (Base 6): +λλλλ.III... years
Earth units (Base 10): 291 years
Earth units (Base 6): 1203 years
Earth units and Eridian numerals (Base 6): IVℓλ years
Eridian units (Base 10): 2517.7 years
Eridian units (Base 6): 15353.4111... years
Eridian units and numerals (Base 6): I∀λ∀λ.+III... years
Length of Rocky and Adrian's Relationship
Earth units (Base 10): 186.3 years
Earth units (Base 6): ~510.2 years
Earth units and Eridian numerals (Base 6): ~∀Iℓ.V years
Eridian units (Base 10): 1611.85 years
Eridian units (Base 6): ~5213.5 years
Eridian units and numerals (Base 6): ~∀VIλ.∀ years
Time Rocky Spent Orbiting Tau Ceti
Earth units (Base 10): 46 years
Earth units (Base 6): 114 years
Earth units and Eridian numerals (Base 6): II+ years
Eridian units (Base 10): ~398 years
Eridian units (Base 6): ~1502 years
Eridian units and numerals (Base 6): I∀ℓV years
All "Earth units" entries obviously use Earth years, Earth seconds, etc. All "Eridian unit" entries use the Eridian equivalents. I've included Base 6 versions of the Earth units, because the book explains that Rocky does all his internal calculations in Base 6 before converting them to Base 10 for Grace. So some of his guesstimates are round numbers in Base 6, but seemingly hyper-specific in Base 10. This is most clear with the Average Eridian Lifespan; 3105 is a much 'cleaner' number than any of the alternatives on the chart.
