8-string tuner spacing

[diegom]

Absolute newbie here!
I have done very basic mods (pickups, tuners, bridges, etc), but this will be my first time doing actual guitar woodwork and I need some guidance.

Given the extremely low price of some eBay basses, I've been tempted to get one to convert to 8-string, for significantly less cost than buying even a used one.

This is the bass in question:
New 34" IB Basswood 24 Frets Electric Bass Guitar Black | eBay
As you can see, it has a relatively large headstock, which should easily accommodate the extra tuners, after some plugging and new drilling.
My question is this, if I were to install the tuners, I would probably be eyeballing it. Is there a better way to do this? Like, actually KNOWING how the tuners should be spaced, instead of just hoping to get it right?

I have an old-fashioned, folded metal bridge with the additional string holes already drilled. For this cheapo attempt, each course will share a saddle. I have already cut quite a few nuts, so I don't foresee this to be a problem. The bass will be strung with octave on top.

Any help or advice on this will be appreciated!

 

[bassburner]

Not a luthier's answer but...

You could find an image of another 8 stringer and based off the known dimensions of the nut (something that's usually published) determine via some maths the spacing of the tuners on that one. An ESP B-208FM would be a good one to look at.

 

[Beej]

@Dadagoboi has posted more about double course basses than anyone else I know.

Check out some of his threads - there are great nuggets of technical information to help guide your choices.
MARCH (MULTISTRING) MADNESS...32 and counting
12 String!

 

[Bruce Johnson]

This calls for some......Engineering! You know, pencil, paper, ruler......and mostly, understanding why the tuners need to be in particular places.

Here's a real good recent thread about this exact subject: Tuner Placement:

Tuner placement

 

[Dadagoboi]

I did it the old fashioned way, bought an 8 string bass and copied its spacing. Did the same for a 12string. But my goal was to make a better multicourse bass and it's good to find out what's been done instead of reinventing the wheel.

Eventually I understood tuner spacings and adapted what I'd used for different headstock shapes.

There's no way to get eight tuners on an SR type headstock.

 

[diegom]

Thank you for all the thoughtful comments!
Like I mentioned, I want to do this "on the cheap". If the bug catches, I will get a proper 8 (or maybe even 12!!) string instrument.
Every one of your comments has made me look at this a bit different. @Bruce Johnson, thank you for the link to the other discussion! That 10-string is a jaw-dropper! Your comments freed me a bit, so I stopped looking at the "straight string pull", which gives me some freedom. @bassburner, of course! How I didn't think of that! Just look at what's been done, and don't try to reinvent the wheel. @Beej, thank you for pointing the path to multistring enlighenment! @Dadagoboi's posts really cleared it all right up! Even he copied the tuner location from another bass!
Which brings me to my mock up. @Dadagoboi, you say there is no way to get 8 tuners in an SR type headstock and I agree. Fortunately got me, the "cheapo" has a LARGE, SR-shaped headstock! A lot more wood than an actual Ibanez...

Without further ado, this is my first mockup in Photoshop. I'm sure there's plenty of room for improvement before I actually attack any wood. No "straight string pull" as of now, the general spacing was copied from
the ESP B-208FM, properly scaled. The white rectangle by the nut is the width of the cheapo, 1.6".
It looks feasible, no?

 

[Dadagoboi]

Which brings me to my mock up. @Dadagoboi, you say there is no way to get 8 tuners in an SR type headstock and I agree. Fortunately got me, the "cheapo" has a LARGE, SR-shaped headstock!

The 8 from four (or 5) SR headstock has been done, slipped my mind. Someone did it maybe 8 years ago with a micro Ibanez fiver, I don't remember the exact thread.

 

[diegom]

The 8 from four (or 5) SR headstock has been done, slipped my mind. Someone did it maybe 8 years ago with a micro Ibanez fiver, I don't remember the exact thread.

Thank you for your input!
I was looking at some of your beautiful builds. If I get comfortable with this 8-string thing, I see a MAX 8 in my horizon...
D.