As one who's hobbies include making furniture from wood.. the best straight cut guide for a circular saw comprises of a straight edge piece of wood and some clamps.
the method is :
with saw unplugged, measure from the edge of the cutting blade to the edge of the bottom plate on your saw..clamp your straight edge this distance away from the intended cut.
remember the blade has two edges, make sure to use the correct edge..could be a difference of 3mm or more in your cut.
even though I have a table saw, band saw, several routers, several compound saws and various other machinery, when it comes to cutting large sheets, I still use the above method..
Hey Hiz,
This is indeed a good method for woodworkers to use. For the average Aussie Home Brewer I have made it even simpler and taken away the need to calculate anything for every cut. Cuts out all the stupid mistakes we all make when goofing one calculation and then mis-cutting an expensive sheet of wood.
Not sure where the previous post went?
Only have the new photos with me so looks like I need to re construct it with just words.
The end of a new sheet of ply (all ends really) are guillotine cut nice and straight.
If we take a slice off the end of a sheet of ply we will get a 1.2 metre long strip of wood with a slightly wobbly side where we cut it by hand and the opposite side will be the nice guillotine straight cut.
To remove all future calculations we use about $4-6 worth of additional wood in the form of 9mm sheet of plywood in my example.
We glue the 1.2 metre stip of wood we cut out to the end of the 9mm sheet of plywood with the STRAIGHT side facing inwards to the CENTRE of the ply. We leave our wobbly cut facing outwards.
After gluing, we get rained on for 5 days straight

and because we have a tiny shed we are forced to work outdoors so we sit inside and drink beer and do other things waiting for a nice day to return
------
Now for the rest of the story!
Put out your three Saw Horses you picked up from the hardware store or built yourself and place the 9mm Sheet of Ply on it as shown.
It is time to remove the clamps as we are done letting the glue dry up and harden completely.
Position your end saw horse near the far edge of the board. When we start cutting we do not want to cut through our saw horse we just bought.
This is the direction we will be cutting. The widest part of your Circular Saw bottom plate will be pressed against the straight side of the thick piece of 1.2 meter long wood we originally cut out. This will act as a guide.
Now cut carefully making sure you put a small amount of pressure to keep the saw up against the straight side nice and flush as you cut down the wood.
Now we are done. You have a very lovely straight cutting jig you can now use to speed up your hive building by a HUGE factor. Not only that you will get straight cuts without needing to buy a Table Saw.
Look how nice and straight and easy it will be to use. Instead of having to do any more calculations again. You simply measure out the wood along each edge to how long you want the piece of cut out wood to be. You line the edge of the cutting jig against the edge mark and simply clamp it down by the thick back portion of the jig. You then do the same to the opposite side and run your Circular Saw down putting the same small amount of pressure pushing the foot plate of the saw against the straight edge of the thick piece of wood. Easy Peasy.
Now instead of calculations to worry about you only have to remember one thing. Keep the fat/thick end of the jig on top of the cut out section. keep the skinny end the blade of the saw runs down pointing towards the centre of the rest of the sheet of wood. Otherwise if you reverse direction you will have to then account for blade thickness and change you line markings by that thickness amount (too much work

)
----------
As an after thought... I then spent the next few hours and almost completely cut up an entire full sheet of 21mm ply for the intermediate hive. 80% done in a little bit of time after "one night home from work" and its almost done. Imagine yourself cutting out two of the horizontal hives already described in this post and already half assembled them in one single night home from work.
Now there is no excuse to not make it in time to build hives for the beginning of the bee season when they swarm in spring.
Cheers,
Brewer Pete