Preview shop online... use the size charts, (some even show you just where to measure) and make a best guess... I'm guessing something in full-figure / plus-size for you, but that depends on if what you're calling "waist", may be factored as 'Hips' in women's wear...
Basically women's panties are not sized by the waist - it is the hips that are more or less the controlling factor.
With your 46 waist I am guessing your hips are not any bigger than maybe a 50 so off the chart I would start
with a size 9 or 10 brief.
Women's hips are generally 4-8 inches bigger than the waist - For Men 2-4 inches - think hour glass figure.
The first time is the most difficult - if you wear jockey 'Y' fronts get a tape measure and the waist UNSTRETCHED
and then the widest part (the hips) and next measure the RISE (distance from center crotch bottom to the front waist band)
Lastly measure across the narrowest part of the crotch (you will need as much room as possible to prevent package fall out)
With these four measurements go to a department store i.e. Wal*Mart would be a good start and look for single
panties out on display and match up your measurements. Buy the cheapest panty that fits those measurements.
After trying the panty for fit if it works then note the size (usually a tag at the back of the waist band or on the left side inseam)
Now you are going somewhere. If the panty is too loose drop down a number in size. Too tight go up one number.
Typically a brief will fit very snug at the waist and just hug the hips.
In your case I would steer away from the bikini or high leg brief - you may not find the fit comfortable.
The full figure brief (aka granny panty) would perhaps be best.
There is another design called a Tap Panty sort of like a man's boxer brief - these do not have much room for the equipment.
It is all trial and error - reboot - restart - so don't be buying sets of 6 or expensive panties till you get it down in your own ball park ! LOL!
I forgot the equation, but my waist is about 32-34 and I wear L size (size 10), i use to wear XL (size 12/14) but I went down in weight. and i've never been bigger than 38... so I think you might need XXL (size 14/18)
The equation that I think you are referring to is the rule of 20.
Take the misses size say a 12 and add 20 to it to get the waist size of 32 inches.
It makes thing confusing when sizing panties - the panties have to be large
enough to go over the hips to rest at the waist (think hourglass figure).
Women's hips are generally proportioned larger than men's - never the less
the under gear has to slide - be pulled - yanked pass the wider hips to get
to home place the waist. The elastic waist band has to have that much stretch
to achieve this.
Also factored in do you like your panties worn tight like compression pants
or loose like granny panties with silky fabric sliding and slipping - even better
if the fabric is double lined (very expensive fantasy panties).
In reality with women's sizes all over the map of the body - one really needs
to experiment around with a trial and error method to see what works.
Panty manufactures do not all agree on one set of standard measurements -
Think "Vanity Factor" in merchandising.
Eventually you will find something that fits - enjoy the search for panty heaven !
I am a 42" waist and find size 8 normally fits me just very slightly on the snug side, which I like. Size 9 is always safe for me. Sizing and fit varies significantly between brands. Trial and error is the best way. T J Max and Ross have the largest selection at the most cost effective price. I don't feel too bad throwing out a $2.00 pair of panties if they don't fit.