These kinds of rules of thumb are always dangerous to use since a bunch of assumptions went into them that are not stated. At worst they can be downright misleading. My advice is to stay away from such "rules". Instead understand what is going on. Then you can derive your own relationships for any specific condition you want, and you'll know what premises they depend on and when they are valid.
The first equation:
Iac = 1.8 Idc
is apparently intended as a quick guide as to the current rating of a transformer feeding a full wave bridge, given the DC current drawn from that bridge. I certainly would never use this in a design. It's basically trying to say that the AC current rating needs to be higher than the ultimate DC current. There is definitely truth in that, since the DC voltage will be roughly the AC peak to peak voltage. Therefore from just a power balance alone, the AC current has to be higher since its RMS voltage is lower than the DC voltage.
But, where exactly did the 1.8 come from? What assumptions about conduction angle into the bridge are embodied into it. What about DC voltage droop? Impedance of the transformer, etc? This equation is useless without knowing what criteria were used to derive it.
Again, when you are faced with sizing a transformer, look at your particular situation and do your own analisys. There is no substitute for knowing what you're doing and then applying that to design decisions.
The second equation:
VA = 1.4 (watts + (2 Idc))
It clearly total garbage from a units check alone. You can't add power and current and expect anything sensible as a result.