0.00000000000000002025 is 21 digits, so obviously you need more than 18 digits.

Using a single byte for the fixed point will get me down to +-1e-127

I think you are misunderstanding "fixed point". Fixed point means the exponent part is always the same. For example, with 64 bit signed integers:

• 0.0000000045 is represented by the integer value 450000000
• Your maximum value of 4.0 would be represented by the integer value 400000000000000000
• Your minimum, -4.0 would be -400000000000000000

But you need more than 18 digits, so you need to cascade. Though I misspoke. What you cascade depends the underlying hardware. For a PC with 64 bit integers, you need to cascade 3 or 4 32 bit integers to your needed digits. For a PC with 32 bit integers, you need to cascade at least 5 16 bit integers.

Let's assume your PC has 64 bit integers. To add 2 numbers, A and B, you split them into Ah, Am, Al, Bh, Bm and Bl, then do the additions:

/* C */
sint64_t A, B, Ah, Am, Al, Bh, Bm, Bl, Ch, Cl, /* etc */;
sint128 C;
Al = A & 0xFFFFFFFF;
Am = A >> 32;
Ah = A >> 64;
Bl = B & 0xFFFFFFFF;
Bm = A >> 32;
Bh = A >> 64;
Cl = Al + Bl;
Cm = Am + Bm + (Cl >> 32);
Ch = Ah + Bh + (Cm >> 32);
C = (Ch << 64) + ((Cm & 0xFFFFFFFF) << 32)  + (Cl & 0xFFFFFFFF);
[download]

Of course, there are some easy optimizations to this I leave as an exercise for the reader. Also leaving subtraction, multiplication and division as exercises for the reader.

Note: sint64_t and sint128_t are the C99 standard type names for "exactly 64/128 bit, signed integer"

Welcome to the world of "bit bashing".