Reminder of the meanings of the terms:
A distinctio ex natura rei is any distinction obtaining apart from the activity of the intellect, including the divine intellect.
A distinctio realis (or distincta realiter) is a distinction between entities that can exist without each other. Probably a subset of the ex natura rei distinction. Sometimes, as in the case of body and soul, only one of the items can exist without the other.
A distinctio formalis obtains ex natura rei but the items so distinguished (definitions, quiddities, formalities, parts of definitions, etc.) are not separable.
Ratio: probably here means definition, or a formal nature.
From Himbertus, Rep IA d. 36 a. 2 (ed D-S, 199-200):
There is a second mode of speaking, which is of our Doctor [=Scotus], that essence and actual existence are not really distinguished. Which is proved thus: whenever some things are really distinct, and one descends from the other, if that which descends is real, then that which remains will be real, as is clear regarding whiteness in a wall; but actual existence descends from essence, and essence remains, and nevertheless is not real; therefore they are not really distinguished.
The second argument: if essence and actual existence are really distinguished, essence will actually exist without actual existence, because whenever some things differ really, one is able to be [esse] without the other; but essence is not able to actually exist without actual existence; therefore they are really the same.
Here are two doubts. It is said that essence is distinguished from actual existence: is it distinguished formally? I say that it is not, because when some things have the same definitional and quidditative ratio, they are the same formally; but essence and actual existence have the same definitional and quidditative ratio; therefore they are the same formally. The major premise is proved, for the formal ratio is taken from the definitional and quidditative ratio. The minor premise is also clear, because neither something else nor a new quiddity is acquired through actual existence.
Second thus: that which does not vary the formal ratio of something does not differ formally from that which it does not vary; but actual existence does not vary the formal ratio of essence; therefore it does not differ formally from it.
The second doubt is if essence and actual existence are distinguished ex natura rei. I say that they are, because whenever it is the case that something befalls one which does not befall the other, those are distinguished ex natura rei, if it befalls them ex natura rei; but it befalls essence that it is not in act, but in potency, and [it befalls] actual existence that it is in act; therefore they are distinguished ex natura rei.
Again, it befalls essence that it is indifferent to being and non being; but actual existence is not indifferent, because it is in act. Whence I say that actual existence and essence are the same really.