Jeff,
Yes, the format you've provided above is the unabreviated 'formal' way of referencing a control on a subform. The following are all legitimate variations of an example that I've put together and tested on a form. See comments later.
(1) =DLookUp("f1","Table1","f3 = [Text0]"

Note: Not really correct. See notes later.
(2) =DLookUp("f1","Table1","f3 = " & [Text0])
(3) =DLookUp("f1","Table1","f3 = " & [Forms]![frmMain]![frmSub].Form!Text0)
(4) =DLookUp("f1","Table1","f3 = " & [Forms]![frmMain]![frmSub].Form("Text0"

)
(5) =DLookUp("f1","Table1","f3 = " & [Forms]![frmMain] ;)](https://cdn.jsdelivr.net/joypixels/assets/8.0/png/unicode/64/1f609.png)
)
(6) =DLookUp("f1","Table1","f3 = " & [Forms]![frmMain]![frmSub].Controls("Text0"

)
Notes:
(a) To set up this example, create a new database with a table called Table1, with two fields f1 (text) and f3 (numeric); record values as follows: a,1; b,2; c,3; d,4; e,5; f,6
(b) Create an unbound form called frmSub with an unbound
text control named Text0, with a default value of 4.
(c) Create 6 more unbound text controls in the above form
and past the 6 formula above (from and including the equals sign) as the controlsource properties of the respective controls.
(d) Open the subform directly. You should note that only the first two computed fields produce a value; the rest (correctly) produce a #name error.
(e) As dz has pointed out, its kindof curious that the first formula works at all. It evaluates the DLookup correctly, even though the whole thing is incorporated as a string. However, try changing the value of the Text0 control from its default value of 4 to say 5. Then press Shift F8 a couple of times to refresh the form. The second control refreshes to the correct lookup value; the first does'nt. Curious that it worked when the form was opened though, (although why would anyone do it this way anyway).
(f) Now create a new form called frmMain and place the subform in it.
(g) Open the new frmMain form, and you should observe that all 6 formula return the same value. Modify the text0 value and they should re-sync together (except the first), demonstrating that all have valid syntax
(h) My own preference is formula (2), which operates correctly on frmSub independent of whether the form is an embedded subform, or a standalone form.
Cheers,
Steve Lewy
Solutions Developer
steve@lewycomputing.com.au
(dont cut corners or you'll go round in circles)