Omitting the relative pronoun "that"

I came across this sentence while surfing on the Internet:

Now I’m calculating how many pages I should do per day (that) would be reasonable.

When read out loud, the sentence sounds sort of natural, but grammatically speaking, it’s wrong because after the verb "to calculate" isn’t a clause but a noun.

Should the word "that" be added back in to the sentence, or is the sentence itself not wrong in the first place?

Thanks for your assistance!