the only problem i see (and this is the one you seem dead set on avoiding) is overdoing it the very first time before you get enough experience to work with the drug and not wanting to do it again. You're trying to make an educated guess with the help of the forum as to how you will personally respond to a chemical that has different effects on everyone, which is, naturally, difficult. Also, it is of little help to say what effects the mushrooms will produce in comparison to the pot (i.e. a person with a low tolerance to marijuana may or may not trip off of a reasonable (1/2 to full eighth) of mushrooms. They're different chemicals, and your brain will not respond the same to them.
I took 2.3g my first trip with my best friends in the forest on the other side of this cemetery, and it was golden. I think that the risk of taking too much definitely outweighs the possibility that you don't take enough. You can always take more during the trip, establish whether or not you're comfortable with the peak as it is, and take a "correcting dose."
I do not see the point of looking at this from a "i want to save enough to trip twice of one eighth" perspective, when you don't have the first one out of the way. If you blaze pot, then you're probably buying an eighth for $30-40 without the slightest thought; having qualms about the efficient use of mushrooms--which are cheaper--seems a little overboard, though understandable, especially if your source is not reliable/you are uncomfortable/unable to acquire more later.
You have also not said anything about your personality, which makes our recommendations even more perilous, considering that we have no basis to predict whether or not psychedelics are even something that you would enjoy. You will arrive at your own dosage level with all the research you do and imput you receive. There is a reason that people say, in general, to start with half the strong dose. The effects should be apparent, and should give you plenty of time to decide whether or not you can handle an even more intense experience. Good luck. Oh, and make sure you have some pot for the comedown. I usually feel like i ate a pot brownie at the end of the trip, so not so much depressed as fucking exhausted.
Any of those who have already tried mushrooms ever get the feeling while tripping like you're sitting in a lawnchair watching some sort of parade, only to realize that it's your life just as usual, only from a different perspective? I seem to get that strange feeling every time...