Is it possible to simulate user editing in Custom Menu?

Hi all,

my system info:

RPASCE Client (JET UI)	RPASCE.25.1.201.0.9.0425.1.202504301344.1659285.1672
RPASCE Server	25.1.201.0.10.3058
Application Config (mfpscs)	25.1.201.0.10.2750

I tried with this:

Custom Menu rule group:
cust_rule_1:
a = if(<condition>, 0, ignore)

Default calc group:
def_calc_rule_1 (there are two expressions in this rule):
a = b - c
b = a + c

But I found that "a" was always calculated by "a = b - c" the expr in def_calc_rule_1. Result from cust_rule_1 will be overridden.
If there is any way I can achieve that:
When Custom Menu (cust_rule_1) is run, and in following default calc group, "a" is seen as edited by system and "b" is forced and calculated.