Random seed within a boundary?

I'm trying to make it so it will return a random number with in a 0 to a (-800) range, How would I do this? This is what I have so far:

while true do
	NUM1:TweenSize(UDim2.new(0, 100,0,math.random()),"Out","Linear",0.05,false)
	wait(0.2)
end

NUM1:TweenSize(UDim2.new(0, 100,0,math.random(-800,0)),"Out","Linear",0.05,false)

If you're the equalizer guy, I wrote you a demo;

-- Put a LocalScript in to a ScreenGui

local num_bars = 30
-- How many bars?
local bar_width = 20
local container_color = BrickColor.new("Institutional white")
local bar_color = BrickColor.new("Really black")
local max_bar_height = 200
local min_bar_height = 3
local tween_speed = .5
-- How fast they go up and down


-- Just something that creates the GUI's 
local function create(instanc)
	return function(info)
		local new = Instance.new(instanc)
		for i,v in pairs(info) do 
			new[i] = v
		end
		return new
	end
end

-- This is your background
local container = create("Frame")
{
	Size = UDim2.new(0,(num_bars*bar_width) + num_bars,0,max_bar_height);
	Position = UDim2.new(.5,-((num_bars*bar_width)/2),.5,-(max_bar_height/2));
	BackgroundTransparency = 0;
	BackgroundColor = container_color;
	BorderSizePixel = 3;
	Parent = script.Parent
}

-- These are your bars
for i = 1, num_bars do 
	local bar = create("Frame")
	{
		Size = UDim2.new(0,bar_width,0,0);
		Position = UDim2.new(0,((i-1)*bar_width) + i,0,container.Size.Y.Offset);
		Name = "bar"..i;
		BorderColor = container_color;
		BorderSizePixel = padding;
		BackgroundColor = bar_color;
		Parent = container
	}
	-- This runs more than one while statement at once
	Spawn(function()
		while true do
			local random_size_y = math.random(min_bar_height, max_bar_height)
			bar:TweenSize(UDim2.new(0,bar_width,0,-random_size_y),"Out", "Linear", tween_speed, true)
			wait(tween_speed)
		end
	end)
end