IK this will confuse you more, but actually, officially, internationally recognized and standardized, you are correct and 1GB = 1000MB = 1000,000,000 B.
Still most people use 1GB for 1024 MBs. Officially, the 1024-based unit system contains a small 'i' before the 'B': thus 1 GiB = 1024 MiB. These units are called
mebibyte and gibibyte (and there are also kibibytes, tebibytes and so on). HDD/SDD/Memory manufacturers use this bureaucratic trick a lot. They sell you for instance a 16GB SD card which contains exactly 16,000,000,000 bytes. When you plug it, you see that it is 14.9 not 16 of your expected units. Funny, but here is the developer of your OS to be blamed. Not the disk/card manufacturer.
Yet for normal, and by normal I mean normal, people 1GB is 1024MB or 1,048,576 KB or 1,073,741,824 B.
And back on topic: whenever I make memory adjustments of things I'm not sure about, I try first by multiplying everything by 2. In your example that would give 3072, 1024 and 256. If that's not satisfactory (and still there is memory to spend) -- repeat with 6144, 2048 and 512 and so on.