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I am not familiar with ADS - however, using a universal simulation program like PSpice it is possible to adjust a certain part within a circuit with feedback for a desired phase margin. This (new) method is described in the following document (chapt. 4).

[URL unfurl="true"]https://link.springer.com/article/10.1007/s00034-012-9437-7[/URL]

The principle of this method is to insert a certain amount of phase shift into the closed loop and adjust a selected part of the circuit until the oscillation condition (zero phase margin) is fulfilled. Then, without this additional phase shift, the circuit has the required margin.

Here is an excerpt from the document:

4 Design Aid for a Specified Margin

Unlike the classical loop-gain-based procedures, the described phase-slope method

also offers the possibility to support the design of a system that requires a specified

phase margin. If both blocks HZ and GL contain a fixed phase shift that is identical to

the required phase margin (ϕZ = ϕPM ), any suitable component of the feedback loop

may be used for parameter stepping in place of ϕZ . However, it can be expected that

there will be a remarkable non-linear relationship between the stepped parameter and

the respective group delay magnitude maximum. Therefore, it is advisable to select

more than just two values for parameter stepping. Again, the desired component value

that ensures the required margin can be found using the goal function in (2).

For example, to provide the circuit in Fig. 1 with a phase margin ϕPM = ϕZ =

60◦ the load capacitor CL was stepped during the ac analysis over the range CL =

(0.1 . . . 1) nF in steps of 0.1 nF. Then, at the zero crossing of the goal function (2), a

value of CL = 0.285 nF can be identified to allow the required margin.

PS: If desired I could provide you (as the author) with a copy of the article (via email)